A005446 -id:A005446 - cp's OEIS frontend

A005447 Numerators of the expansion of -W_{-1}(-e^(-1 - x^2/2)) where x > 0 and W_{-1} is the Lambert W function.

1, 1, 1, 1, -1, 1, 1, -139, 1, -571, -281, 163879, -5221, 5246819, 5459, -534703531, 91207079, -4483131259, -2650986803, 432261921612371, -6171801683, 6232523202521089, 4283933145517, -25834629665134204969, 11963983648109

Offset: 0

N. J. A. Sloane

Numerators of the expansion of -W_0(-e^(-1 - x^2/2)) where x < 0 an W_0 is the principal branch of the Lambert W function. - Michael Somos, Oct 06 2017

See A299430/A299431 for more formulas; given g.f. A(x) = Sum_{n>=0} A005447(n)/A005446(n)*x^n, then A(x)^2 = Sum_{n>=0} A299430(n)/A299431(n)*x^n.

			G.f.: A(x) = 1 + x + (1/3)*x^2 + (1/36)*x^3 - (1/270)*x^4 + (1/4320)*x^5 + (1/17010)*x^6 - (139/5443200)*x^7 + (1/204120)*x^8 + ... + (A005447(n)/A005446(n))*x^n + ...

E. T. Copson, An Introduction to the Theory of Functions of a Complex Variable, 1935, Oxford University Press, p. 221.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Vincenzo Librandi, Table of n, a(n) for n = 0..100
J. M. Borwein and R. M. Corless, Emerging Tools for Experimental Mathematics, Amer. Math. Monthly, 106 (No. 10, 1999), 889-909.
G. Marsaglia and J. C. W. Marsaglia, A new derivation of Stirling's approximation to n!, Amer. Math. Monthly, 97 (1990), 827-829.
NIST Digital Library of Mathematical Functions, Lambert W-Function, section 4.13.7
J. C. W. Marsaglia, The incomplete gamma function and Ramanujan's rational approximation to exp(x), J. Statist. Comput. Simulation, 24 (1986), 163-168. [_N. J. A. Sloane_, Jun 23 2011]

Cf. A005446 (denominators), A090804/A065973.

Cf. A299430 / A299431 (A(x)^2), A299432 / A299433.

h := proc(k) option remember; local j; `if`(k<=0,1,(h(k-1)/k-add((h(k-j)*h(j))/(j+1),j=1..k-1))/(1+1/(k+1))) end:
A005447 := n -> `if`(n<4,1,`if`(n=4,-1,numer(h(n-1))));
seq(A005447(i),i=0..24); # Peter Luschny, Feb 08 2011
# other program
a[1]:=1;
M:=25;
for n from 2 to M do
t1:=a[n-1]/(n+1)-add(a[k]*a[n+1-k],k=2..floor(n/2));
if n mod 2 = 1 then t1:=t1-a[(n+1)/2]^2/2; fi;
a[n]:=t1;
od:
s1:=[seq(a[n],n=1..M)]; # N. J. A. Sloane, Jun 23 2011, based on J. Marsaglia's 1986 paper

terms = 25; Assuming[x > 0, -ProductLog[-1, -Exp[-1 - x^2/2]] + O[x]^terms] // CoefficientList[#, x]& // Take[#, terms]& // Numerator (* Jean-François Alcover, Jun 20 2013, updated Feb 21 2018 *)
a[ n_] := If[ n < 0, 0, Block[ {$Assumptions = x < 0}, SeriesCoefficient[ -ProductLog[ -Exp[-1 - x^2/2]], {x, 0, n}] // Numerator]]; (* Michael Somos, Oct 06 2017 *)

{a(n) = my(A); if( n<1, n==0, A = vector(n, k, 1); for(k=2, n, A[k] = (A[k-1] - sum(i=2, k-1, i * A[i] * A[k+1-i])) / (k+1)); numerator(A[n]))}; /* Michael Somos, Jun 09 2004 */

{a(n) = if( n<1, n==0, numerator( polcoeff( serreverse(sqrt( 2 * (x - log(1 + x + x^2 * O(x^n))))), n)))}; /* Michael Somos, Jun 09 2004 */

@CachedFunction
def h(n): return 1 if (n<1) else ((n+1)/(n+2))*( h(n-1)/n - sum( h(n-j)*h(j)/(j+1) for j in range(1,n) ))
def A005447(n):
    if (n<4): return 1
    elif (n==4): return -1
    else: return numerator(h(n-1))
[A005447(n) for n in range(31)] # G. C. Greubel, Nov 21 2022

G.f.: A(x) = Sum_{n>=0} A005447(n)/A005446(n)*x^n satisfies log(A(x)) = A(x) - 1 - x^2/2.

Edited by Michael Somos, Jul 21 2002

A065973 Denominators in an asymptotic expansion of Ramanujan.

Original entry on oeis.org

3, 135, 2835, 8505, 12629925, 492567075, 1477701225, 39565450299375, 2255230667064375, 6765692001193125, 7002491221234884375, 21007473663704653125, 441156946937797715625, 56995271759628775870171875

Offset: 0

N. J. A. Sloane, Dec 09 2001

			-2/3, 4/135, -8/2835, -16/8505, 8992/12629925, 334144/492567075, -698752/1477701225, ...

G. E. Andrews, R. Askey and R. Roy, Special Functions, Cambridge, 1999; Problem 4, p. 616.
B. C. Berndt, Ramanujan's Notebooks II, Springer, 1989; p. 181, Entry 48. See also pp. 184, 193ff.
E. T. Copson, An Introduction to the Theory of Functions of a Complex Variable, Oxford Univ. Press, 1935; see p. 230, Problem 18.
S. Ramanujan, Collected Papers, edited by G. H. Hardy et al., Cambridge, 1927, pp. 323-324, Question 294.

Robert Israel, Table of n, a(n) for n = 0..320 (0 .. 126 from G. C. Greubel and D. Turner)
J. C. W. Marsaglia, The incomplete gamma function and Ramanujan's rational approximation to exp(x), J. Statist. Comput. Simulation, 24 (1986), 163-168.
Cormac O'Sullivan, Ramanujan's approximation to the exponential function and generalizations, arXiv:2205.08504 [math.NT], 2022.

Cf. A260306 (numerators), A090804, A005446, A005447.

# Maple program from N. J. A. Sloane, Jun 23 2011, based on J. Marsaglia's 1986 paper:
a[1]:=1;
M:=20;
for n from 2 to M do
t1:=a[n-1]/(n+1)-add(a[k]*a[n+1-k],k=2..floor(n/2));
if n mod 2 = 1 then t1:=t1-a[(n+1)/2]^2/2; fi;
a[n]:=t1;
od:
s1:=[seq(a[n],n=1..M)]: # This gives A005447/A005446
s2:=[seq(-2^(n+1)*(n+1)!*a[2*n+2],n=0..(M-2)/2)]: # This gives A090804/A065973
map(denom,s2);

Denominator[Table[2^n*(3*n + 2)! * Sum[ Sum[ (-1)^(j + 1)*2^i*StirlingS2[2*n + i + j + 1, j]/((2*n + i + j + 1)!*(2*n - i + 1)!*(i - j)!*(n + i + 1)), {j, 1, i}], {i, 1, 2*n+1}], {n, 0, 20}]] (* Vaclav Kotesovec, Nov 20 2015 *)

a(n)=local(A,m); if(n<0,0,n++; A=vector(m=2*n,k,1); for(k=2,m,A[k]=(A[k-1]-sum(i=2,k-1,i*A[i]*A[k+1-i]))/(k+1)); denominator(A[m]*2^n*n!)) /* Michael Somos, Jun 09 2004 */

Define t_n by Sum_{k=0..n-1} n^k/k! + t_n*n^n/n! = exp(n)/2; then t_n ~ 1/3 + 4/(135*n) - 8/(2835*n^2) + ...

Integral_{0..infinity} exp(-x)*(1+x/n)^n dx = exp(n)*Gamma(n+1)/(2*n^n) + 2/3 - 4/(135*n) + 8/(2835*n^2) + 16/(8505*n^3) - 8992/(12629925*n^4) + ...

Maple program edited by Robert Israel, Dec 15 2015

A299430 Numerators of coefficients in C(x) where: C(x)^(1/2) - S(x)^(1/2) = 1 such that C'(x)S(x)^(1/2) = S'(x)C(x)^(1/2) = 2xC(x).

Original entry on oeis.org

1, 2, 5, 13, 43, 5, -19, 41, 1, -7243, 923, 23183, -21401, 64243259, -73, -471741703, 328578883, -11162934047, -103170103, 405632121933911, -811862551, 6065578925646109, 2180051549129, -261709011005693843, 36779766732769, -1552304807519349743393, -33230318647573, 727642520296392573166003, -85602927913097260249, 3885938896026347271301517

Offset: 0

Paul D. Hanna, Feb 09 2018

Given g.f. C(x) = Sum_{n>=0} A299430(n)/A299431(n)*x^n, then C(x)^(1/2) = Sum_{n>=0} A005447(n)/A005446(n)*x^n.

			G.f.: C(x) = 1 + 2*x + 5/3*x^2 + 13/18*x^3 + 43/270*x^4 + 5/432*x^5 - 19/17010*x^6 + 41/2721600*x^7 + 1/40824*x^8 - 7243/1175731200*x^9 + 923/1515591000*x^10 + ...
Related power series begin:
S(x) = x^2 + 2/3*x^3 + 1/6*x^4 + 1/90*x^5 - 1/810*x^6 + 1/15120*x^7 + 1/68040*x^8 - 139/24494400*x^9 + 1/1020600*x^10 - 571/12933043200*x^11 + ...
sqrt(C) = 1 + x + 1/3*x^2 + 1/36*x^3 - 1/270*x^4 + 1/4320*x^5 + 1/17010*x^6 - 139/5443200*x^7 + 1/204120*x^8 - 571/2351462400*x^9 - 281/1515591000*x^10 + ... + A005447(n)/A005446(n)*x^n + ...

Cf. A299431 (denominators in C), A299432/A299433 (S), A005447/A005446 (sqrt(C)).

terms = 30; Assuming[x>0, ProductLog[-1, -Exp[-1 - x^2/2]]^2 + O[x]^terms] // CoefficientList[#, x]& // Numerator (* Jean-François Alcover, Feb 22 2018 *)

{a(n) = my(C=1, S=x^2); for(i=0,n, C = (1 + sqrt(S +O(x^(n+2))))^2; S = intformal( 2*x*sqrt(C) ) ); numerator(polcoeff(C,n))}
for(n=0,30,print1(a(n),", "))

The functions C = C(x) and S = S(x) satisfy:
(1) sqrt(C) - sqrt(S) = 1.
(2a) C'*sqrt(S) = S'*sqrt(C) = 2*x*C.
(2b) C' = 2*x*C/sqrt(S).
(2c) S' = 2*x*sqrt(C).
(3a) C = 1 + Integral 2*x*C/sqrt(S) dx.
(3b) S = Integral 2*x*sqrt(C) dx.
(4a) sqrt(C) = exp( Integral x/(sqrt(C) - 1) dx ).
(4b) sqrt(S) = exp( Integral x/sqrt(S) dx ) - 1.
(5a) C - S = exp( Integral 2*x*C/(C*sqrt(S) + S*sqrt(C)) dx ).
(5b) C - S = exp( Integral C'*S'/(C*S' + S*C') dx).
(6a) sqrt(C) = exp( sqrt(C) - 1 - x^2/2 ).
(6b) sqrt(C) = 1 + x^2/2 + Integral x/(sqrt(C) - 1) dx.

A299431 Denominators of coefficients in C(x) where: C(x)^(1/2) - S(x)^(1/2) = 1 such that C'(x)S(x)^(1/2) = S'(x)C(x)^(1/2) = 2xC(x).

Original entry on oeis.org

1, 1, 3, 18, 270, 432, 17010, 2721600, 40824, 1175731200, 1515591000, 217275125760, 354648294000, 5084237942784000, 114578679600, 915162829701120000, 1595278956070800000, 298709147614445568000, 818378104464320400000, 168561571998831634022400000, 982053725357184480000, 45745017385529077294694400000, 279517041579788632620000000

Offset: 0

Paul D. Hanna, Feb 09 2018

Given g.f. C(x) = Sum_{n>=0} A299430(n)/A299431(n)*x^n, then C(x)^(1/2) = Sum_{n>=0} A005447(n)/A005446(n)*x^n.

			G.f.: C(x) = 1 + 2*x + 5/3*x^2 + 13/18*x^3 + 43/270*x^4 + 5/432*x^5 - 19/17010*x^6 + 41/2721600*x^7 + 1/40824*x^8 - 7243/1175731200*x^9 + 923/1515591000*x^10 + ...
Related power series begin:
S(x) = x^2 + 2/3*x^3 + 1/6*x^4 + 1/90*x^5 - 1/810*x^6 + 1/15120*x^7 + 1/68040*x^8 - 139/24494400*x^9 + 1/1020600*x^10 - 571/12933043200*x^11 + ...
sqrt(C) = 1 + x + 1/3*x^2 + 1/36*x^3 - 1/270*x^4 + 1/4320*x^5 + 1/17010*x^6 - 139/5443200*x^7 + 1/204120*x^8 - 571/2351462400*x^9 - 281/1515591000*x^10 + ...
+ A005447(n)/A005446(n)*x^n + ...

Cf. A299430 (numerators in C), A299432/A299433 (S), A005447/A005446 (sqrt(C)).

terms = 30; Assuming[x>0, ProductLog[-1, -Exp[-1 - x^2/2]]^2 + O[x]^terms] // CoefficientList[#, x]& // Denominator (* Jean-François Alcover, Feb 22 2018 *)

{a(n) = my(C=1, S=x^2); for(i=0,n, C = (1 + sqrt(S +O(x^(n+2))))^2; S = intformal( 2*x*sqrt(C) ) ); denominator(polcoeff(C,n))}
for(n=0,30,print1(a(n),", "))

The functions C = C(x) and S = S(x) satisfy:
(1) sqrt(C) - sqrt(S) = 1.
(2a) C'*sqrt(S) = S'*sqrt(C) = 2*x*C.
(2b) C' = 2*x*C/sqrt(S).
(2c) S' = 2*x*sqrt(C).
(3a) C = 1 + Integral 2*x*C/sqrt(S) dx.
(3b) S = Integral 2*x*sqrt(C) dx.
(4a) sqrt(C) = exp( Integral x/(sqrt(C) - 1) dx ).
(4b) sqrt(S) = exp( Integral x/sqrt(S) dx ) - 1.
(5a) C - S = exp( Integral 2*x*C/(C*sqrt(S) + S*sqrt(C)) dx ).
(5b) C - S = exp( Integral C'*S'/(C*S' + S*C') dx).
(6a) sqrt(C) = exp( sqrt(C) - 1 - x^2/2 ).
(6b) sqrt(C) = 1 + x^2/2 + Integral x/(sqrt(C) - 1) dx.

A090804 Numerators in an asymptotic expansion of Ramanujan.

Original entry on oeis.org

2, -4, 8, 16, -8992, -334144, 698752, 23349012224, -1357305243136, -6319924923392, 8773495082018816, 49004477022654464, -1709650943378038784, -480380831834367035260928, 88481173388026066736939008, 660883915180095254454665216

Offset: 0

N. J. A. Sloane, Feb 11 2004

2^(n+1) divides a(n). - Peter Luschny, Nov 05 2015

			2/3, -4/135, 8/2835, 16/8505, -8992/12629925, -334144/492567075, 698752/1477701225, ...

G. E. Andrews, R. Askey and R. Roy, Special Functions, Cambridge, 1999; Problem 4, p. 616.
B. C. Berndt, Ramanujan's Notebooks II, Springer, 1989; p. 181, Entry 48. See also pp. 184, 193ff.
E. T. Copson, An Introduction to the Theory of Functions of a Complex Variable, Oxford Univ. Press, 1935; see p. 230, Problem 18.
S. Ramanujan, Collected Papers, edited by G. H. Hardy et al., Cambridge, 1927, pp. 323-324, Question 294.

G. C. Greubel, Table of n, a(n) for n = 0..160
J. D. Buckholtz, Concerning an approximation of Copson, Proc. Amer. Math. Soc., 14 (1963), 564-568.
K. P. Choi, On the medians of gamma distributions and an equation of Ramanujan, Proceedings of the American Mathematical Society 121:1 (May, 1994), pp. 245-251.
J. C. W. Marsaglia, The incomplete gamma function and Ramanujan's rational approximation to exp(x), J. Statist. Comput. Simulation, 24 (1986), 163-168. [From _N. J. A. Sloane_, Jun 23 2011]
Cormac O'Sullivan, Ramanujan's approximation to the exponential function and generalizations, arXiv:2205.08504 [math.NT], 2022.

Cf. A065973 (denominators), A005446, A005447, A264148.

# Maple program from N. J. A. Sloane, Jun 23 2011, based on J. Marsaglia's 1986 paper:
a[1]:=1;
M:=20;
for n from 2 to M do
t1:=a[n-1]/(n+1)-add(a[k]*a[n+1-k],k=2..floor(n/2));
if n mod 2 = 1 then t1:=t1-a[(n+1)/2]^2/2; fi;
a[n]:=t1;
od:
s1:=[seq(a[n],n=1..M)]; # This gives A005447/A005446
s2:=[seq(2^(n+1)*(n+1)!*a[2*n+2],n=0..M/2)]; # This gives A090804/A065973
# minor corrections by Peter Luschny, Nov 05 2015

Numerator@Table[(n+1)! SeriesCoefficient[-(ProductLog[-1, -Exp[-x^2-1]] + ProductLog[-Exp[-x^2-1]] + 1)/(2 x^2), {x, 0, 2n}, Assumptions -> x>0], {n, 0, 15}] (* Vladimir Reshetnikov, Nov 09 2015 *)
Numerator@Table[KroneckerDelta[n] + 2^n (3n+2)! Sum[((-1)^j 2^i StirlingS2[2n+i+j+1, j])/((2n+i+j+1)! (2n-i+1)! (i-j)! (n+i+1)), {i, 1, 2n+1}, {j, 1, i}], {n, 0, 15}]  (* Vladimir Reshetnikov, Nov 09 2015 *)

{a(n) = my(A, m); if( n<1, n==0, n++; A = vector(m=2*n, k, 1); for(k=2, m, A[k] = (A[k-1] - sum(i=2, k-1, i * A[i] * A[k+1-i])) / (k + 1)); numerator(A[m] * 2^n * n!))}; /* Michael Somos, Jun 09 2004 */

Define t_n by Sum_{k=0..n-1} n^k/k! + t_n*n^n/n! = exp(n)/2; then t_n ~ 1/3 + 4/(135*n) - 8/(2835*n^2) + ...
Integral_{0..infinity} exp(-x)*(1+x/n)^n dx = exp(n)*Gamma(n+1)/(2*n^n) + 2/3 - 4/(135*n) + 8/(2835*n^2) + 16/(8505*n^3) - 8992/(12629925*n^4) + ...
From Vladimir Reshetnikov, Nov 09 2015: (Start)
Numerators/denominators: A090804(n)/A065973(n) = 0^n + 2^n * (3*n+2)! / (2*n+1)! * Sum_{i=1..2*n+1} Sum_{j=1..i} Sum_{k=1..j} (-1)^k * 2^i * k^(2*n+i+j+1) * C(2*n+1,i) * C(i,j) * C(j,k) / ((2*n+i+j+1)! * (n+i+1)), where C(n,k) = A007318(n,k) are binomial coefficients.
A090804(n)/A065973(n) = 0^n + 2^n * (3*n+2)! * Sum_{i=1..2*n+1} Sum_{j=1..i} (-1)^j * 2^i * stirling2(2*n+i+j+1,j) / ((2*n+i+j+1)! * (2*n-i+1)! * (i-j)! * (n+i+1)).
(End)

a(0) corrected by Peter Luschny, Nov 05 2015

A299432 Numerators of coefficients in S(x) where: C(x)^(1/2) - S(x)^(1/2) = 1 such that C'(x)S(x)^(1/2) = S'(x)C(x)^(1/2) = 2xC(x).

Original entry on oeis.org

0, 0, 1, 2, 1, 1, -1, 1, 1, -139, 1, -571, -281, 163879, -5221, 5246819, 5459, -534703531, 91207079, -4483131259, -2650986803, 432261921612371, -6171801683, 6232523202521089, 4283933145517, -25834629665134204969, 11963983648109, -1579029138854919086429, -208697624924077, 746590869962651602203151, -29320119130515566117

Offset: 0

Paul D. Hanna, Feb 09 2018

			G.f.: S(x) = x^2 + 2/3*x^3 + 1/6*x^4 + 1/90*x^5 - 1/810*x^6 + 1/15120*x^7 + 1/68040*x^8 - 139/24494400*x^9 + 1/1020600*x^10 - 571/12933043200*x^11 + ...
Related power series begin:
C(x) = 1 + 2*x + 5/3*x^2 + 13/18*x^3 + 43/270*x^4 + 5/432*x^5 - 19/17010*x^6 + 41/2721600*x^7 + 1/40824*x^8 - 7243/1175731200*x^9 + 923/1515591000*x^10 + ...
sqrt(C) = 1 + x + 1/3*x^2 + 1/36*x^3 - 1/270*x^4 + 1/4320*x^5 + 1/17010*x^6 - 139/5443200*x^7 + 1/204120*x^8 - 571/2351462400*x^9 - 281/1515591000*x^10 + ... + A005447(n)/A005446(n)*x^n + ...

Cf. A299433 (denominators in S), A299430/A299431 (C), A005447/A005446 (sqrt(C)).

terms = 30; c[x_] = Assuming[x > 0, ProductLog[-1, -Exp[-1 - x^2/2]]^2 + O[x]^terms]; Integrate[2*x*Sqrt[c[x]] + O[x]^terms, x] // CoefficientList[#, x] & // Numerator (* Jean-François Alcover, Feb 22 2018 *)

{a(n) = my(C=1, S=x^2); for(i=0,n, C = (1 + sqrt(S +O(x^(n+2))))^2; S = intformal( 2*x*sqrt(C) ) ); numerator(polcoeff(S,n))}
for(n=0,30,print1(a(n),", "))

The functions C = C(x) and S = S(x) satisfy:
(1) sqrt(C) - sqrt(S) = 1.
(2a) C'*sqrt(S) = S'*sqrt(C) = 2*x*C.
(2b) C' = 2*x*C/sqrt(S).
(2c) S' = 2*x*sqrt(C).
(3a) C = 1 + Integral 2*x*C/sqrt(S) dx.
(3b) S = Integral 2*x*sqrt(C) dx.
(4a) sqrt(C) = exp( Integral x/(sqrt(C) - 1) dx ).
(4b) sqrt(S) = exp( Integral x/sqrt(S) dx ) - 1.
(5a) C - S = exp( Integral 2*x*C/(C*sqrt(S) + S*sqrt(C)) dx ).
(5b) C - S = exp( Integral C'*S'/(C*S' + S*C') dx).
(6a) sqrt(C) = exp( sqrt(C) - 1 - x^2/2 ).
(6b) sqrt(C) = 1 + x^2/2 + Integral x/(sqrt(C) - 1) dx.

A299433 Denominators of coefficients in S(x) where: C(x)^(1/2) - S(x)^(1/2) = 1 such that C'(x)S(x)^(1/2) = S'(x)C(x)^(1/2) = 2xC(x).

Original entry on oeis.org

1, 1, 1, 3, 6, 90, 810, 15120, 68040, 24494400, 1020600, 12933043200, 9093546000, 14122883174400, 2482538058000, 76263569141760000, 59580913392000, 15557768104919040000, 14357510604637200000, 28377369023372328960000, 8183781044643204000000, 3539793011975464314470400000, 270064774473225732000000, 13677760198273194111113625600000

Offset: 0

Paul D. Hanna, Feb 09 2018

			G.f.: S(x) = x^2 + 2/3*x^3 + 1/6*x^4 + 1/90*x^5 - 1/810*x^6 + 1/15120*x^7 + 1/68040*x^8 - 139/24494400*x^9 + 1/1020600*x^10 - 571/12933043200*x^11 + ...
Related power series begin:
C(x) = 1 + 2*x + 5/3*x^2 + 13/18*x^3 + 43/270*x^4 + 5/432*x^5 - 19/17010*x^6 + 41/2721600*x^7 + 1/40824*x^8 - 7243/1175731200*x^9 + 923/1515591000*x^10 + ...
sqrt(C) = 1 + x + 1/3*x^2 + 1/36*x^3 - 1/270*x^4 + 1/4320*x^5 + 1/17010*x^6 - 139/5443200*x^7 + 1/204120*x^8 - 571/2351462400*x^9 - 281/1515591000*x^10 + ... + A005447(n)/A005446(n)*x^n + ...

Cf. A299432 (numerators in S), A299430/A299431 (C), A005447/A005446 (sqrt(C)).

terms = 30; c[x_] = Assuming[x > 0, ProductLog[-1, -Exp[-1 - x^2/2]]^2 + O[x]^terms]; Integrate[2*x*Sqrt[c[x]] + O[x]^terms, x] // CoefficientList[#, x] & // Denominator (* Jean-François Alcover, Feb 22 2018 *)

{a(n) = my(C=1, S=x^2); for(i=0,n, C = (1 + sqrt(S +O(x^(n+2))))^2; S = intformal( 2*x*sqrt(C) ) ); denominator(polcoeff(S,n))}
for(n=0,30,print1(a(n),", "))

The functions C = C(x) and S = S(x) satisfy:
(1) sqrt(C) - sqrt(S) = 1.
(2a) C'*sqrt(S) = S'*sqrt(C) = 2*x*C.
(2b) C' = 2*x*C/sqrt(S).
(2c) S' = 2*x*sqrt(C).
(3a) C = 1 + Integral 2*x*C/sqrt(S) dx.
(3b) S = Integral 2*x*sqrt(C) dx.
(4a) sqrt(C) = exp( Integral x/(sqrt(C) - 1) dx ).
(4b) sqrt(S) = exp( Integral x/sqrt(S) dx ) - 1.
(5a) C - S = exp( Integral 2*x*C/(C*sqrt(S) + S*sqrt(C)) dx ).
(5b) C - S = exp( Integral C'*S'/(C*S' + S*C') dx).
(6a) sqrt(C) = exp( sqrt(C) - 1 - x^2/2 ).
(6b) sqrt(C) = 1 + x^2/2 + Integral x/(sqrt(C) - 1) dx.

A264148 Numerators of rational coefficients related to Stirling's asymptotic series for the Gamma function.

Original entry on oeis.org

1, 2, 1, -4, 1, 8, -139, 16, -571, -8992, 163879, -334144, 5246819, 698752, -534703531, 23349012224, -4483131259, -1357305243136, 432261921612371, -6319924923392, 6232523202521089, 8773495082018816, -25834629665134204969, 49004477022654464, -1579029138854919086429

Offset: 0

Peter Luschny, Nov 05 2015

The rational numbers SGGS = A264148/A264149 (SGGS stands for 'Stirling Generalized Gamma Series') are a supersequence of the coefficients in Stirling's asymptotic series for the Gamma function A001163/A001164 and of an asymptotic expansion of Ramanujan A090804/A065973, further they appear in scaled form in an expansion of -W_{-1}(-e^{-1-x^2/2}) where W_{-1} is Lambert W function A005447/A005446.

Ramanujan's asymptotic expansion theta(n) = 1/3+4/(135n)-8/(2835n^2)- ... is considered in the literature also in the form 1-theta(n) (see for example formula (5) in the Choi link). It is this form to which we refer here.

G. C. Greubel, Table of n, a(n) for n = 0..360
K. P. Choi, On the medians of gamma distributions and an equation of Ramanujan, Proceedings of the American Mathematical Society 121:1 (May, 1994), pp. 245-251. [From Vladimir Reshetnikov]
G. Nemes, On the coefficients of the asymptotic expansion of n!, J. Integer Seqs. 13 (2010), 5. [From Vladimir Reshetnikov]

A264148(n) = numerator(SGGS(n)).
A264149(n) = denominator(SGGS(n)).
A001163(n) = numerator(SGGS(2*n)) = numerator(SGGS(2*n)/2^(n+1)).
A001164(n) = denominator(SGGS(2*n)).
A090804(n) = numerator(SGGS(2*n+1)).
A065973(n) = denominator(SGGS(2*n+1)) = denominator(SGGS(2*n+1)/2^(n+1)).
A005447(n+1) = numerator(SGGS(n)/2^(n+1)).
A264150(n) = numerator(SGGS(2*n+1)/2^(n+1)).

h := proc(k) option remember; local j; `if`(k<=0, 1,
(h(k-1)/k-add((h(k-j)*h(j))/(j+1), j=1..k-1))/(1+1/(k+1))) end:
SGGS := n -> h(n)*doublefactorial(n-1):
A264148 := n -> numer(SGGS(n)): seq(A264148(n), n=0..24);

h[k_] := h[k] = If[k <= 0, 1, (h[k - 1]/k - Sum[h[k - j]*h[j]/(j + 1), {j, 1, k - 1}]) / (1 + 1/(k + 1))]; a[n_] := h[n]* Factorial2[n - 1] // Numerator; Table[a[n], {n, 0, 24}]

def A264148(n):
    @cached_function
    def h(k):
        if k<=0: return 1
        S = sum((h(k-j)*h(j))/(j+1) for j in (1..k-1))
        return (h(k-1)/k-S)/(1+1/(k+1))
    return numerator(h(n)*(n-1).multifactorial(2))
print([A264148(n) for n in (0..17)])

Let SGGS(n) = h(n)*doublefactorial(n-1) where h(n) = 1 for n<=0 and for n>0 defined by the recurrence (h(k-1)/k - Sum_{j=1..k-1}((h(k-j)*h(j))/(j+1))/ (1+1/(k+1))) then a(n) = numerator(SGGS(n)).

A260306 Numerators in Ramanujan's asymptotic expansion of theta(n), defined by Sum_{k=0..n-1} n^k/k! + theta(n)*n^n/n! = exp(n)/2.

Original entry on oeis.org

1, 4, -8, -16, 8992, 334144, -698752, -23349012224, 1357305243136, 6319924923392, -8773495082018816, -49004477022654464, 1709650943378038784, 480380831834367035260928, -88481173388026066736939008, -660883915180095254454665216, 888962079683152174584309088256

Offset: 0

Vladimir Reshetnikov, Nov 10 2015

Let Sum_{k=0..n-1} n^k/k! + theta(n)*n^n/n! = exp(n)/2. Ramanujan gave initial terms of the asymptotic expansion theta(n) = 1/3 + (4/135)/n + (-8/2835)/n^2 + (-16/8505)/n^3 + O(1/n^4). This sequence gives the numerators in this expansion, and A065973 gives the denominators.

Ramanujan's asymptotic is also considered in the literature in the form 1-theta(n) (see for example formula (5) in the Choi link). The numerators that appear in that form are given in A090804 (the denominators are the same). The first term A090804(0) = 2 is different, and signs of other terms are opposite to a(n).

G. E. Andrews, R. Askey and R. Roy, Special Functions, Cambridge, 1999; Problem 4, p. 616.
B. C. Berndt, Ramanujan's Notebooks II, Springer, 1989; p. 181, Entry 48. See also pp. 184, 193ff.
E. T. Copson, An Introduction to the Theory of Functions of a Complex Variable, Oxford Univ. Press, 1935; see p. 230, Problem 18.
S. Ramanujan, Collected Papers, edited by G. H. Hardy et al., Cambridge, 1927, pp. 323-324, Question 294.

G. C. Greubel and D. Turner, Table of n, a(n) for n = 0..117
K. P. Choi, On the medians of gamma distributions and an equation of Ramanujan, Proceedings of the American Mathematical Society 121:1 (May, 1994), pp. 245-251.
J. C. W. Marsaglia, The incomplete gamma function and Ramanujan's rational approximation to exp(x), J. Statist. Comput. Simulation, 24 (1986), 163-168.
Cormac O'Sullivan, Ramanujan's approximation to the exponential function and generalizations, arXiv:2205.08504 [math.NT], 2022.

Cf. A065973 (denominators), A090804, A264148, A005447, A005446.

h := proc(k) option remember; local j; `if`(k<=0, 1,
(h(k-1)/k-add((h(k-j)*h(j))/(j+1),j=1..k-1))/(1+1/(k+1))) end:
A260306 := n -> `if`(n=0, 1, -numer(h(2*n+1)*doublefactorial(2*n))):
seq(A260306(n), n=0..16); # Peter Luschny, Nov 20 2015

Numerator[Table[2^n*(3*n + 2)! * Sum[ Sum[ (-1)^(j + 1)*2^i*StirlingS2[2*n + i + j + 1, j]/((2*n + i + j + 1)!*(2*n - i + 1)!*(i - j)!*(n + i + 1)), {j, 1, i}], {i, 1, 2*n+1}], {n, 0, 20}]] (* Vaclav Kotesovec, Nov 20 2015 *)

Numerators/denominators: a(n)/A065973(n) = 2^n * (3*n+2)! / (2*n+1)! * Sum_{i=1..2*n+1} Sum_{j=1..i} Sum_{k=1..j} (-1)^(k+1) * 2^i * k^(2*n+i+j+1) * C(2*n+1,i) * C(i,j) * C(j,k) / ((2*n+i+j+1)! * (n+i+1)), where C(n,k) = A007318(n,k) are binomial coefficients.

a(n)/A065973(n) = 2^n * (3*n+2)! * Sum_{i=1..2*n+1} Sum_{j=1..i} (-1)^(j+1) * 2^i * stirling2(2*n+i+j+1,j) / ((2*n+i+j+1)! * (2*n-i+1)! * (i-j)! * (n+i+1)).

More terms from Vaclav Kotesovec, Nov 20 2015

A177677 The maximum integer dimension in which the volume of the hypersphere of radius n remains larger than 1.

Original entry on oeis.org

12, 62, 147, 266, 419, 607, 828, 1084, 1375, 1699, 2057, 2450, 2877, 3338, 3833, 4362, 4926, 5523, 6155, 6821, 7521, 8256, 9024, 9827, 10664, 11535, 12440, 13379, 14353, 15360, 16402, 17478, 18588, 19732, 20911, 22123, 23370, 24651, 25966, 27315

Offset: 1

Michel Lagneau, May 10 2010

nonn

The volume of the d-dimensional hypersphere of radius n is V= Pi^(d/2) * n^d / Gamma(1 + d/2).

For fixed radius, V -> 0 as d->infinity, so there is a dimension d for which V(n,d) > 1 but V(n,d+1) < 1, which defines the entry in the sequence.

			a(n=2)=62 because Pi^(62/2) * 2^62/GAMMA(1 + (62/2)) =1.447051 and Pi^(63/2)* 2^63 / Gamma(1 + (63/2)) =0.9103541.

Eric Weisstein, Stirling Series, MathWorld.
Wikipedia, Hypersphere

Cf. A005446 , A005147, A001164, A005146, A005447, A001163

with(numtheory): n0:=50: T:=array(1..n0): for r from 1 to n0 do: x:=2: for n from 1 to 1000000 while(x>=1) do: x:= floor(evalf((r^n * Pi^(n/2))/GAMMA(1 + n/2))):k:=n:od:T[r]:=k-1:od:print(T):

a(n) = max {d: Pi^d/2 * n^d / Gamma(1+d/2) > 1}.

Use of variables standardized. Definition simplified, comments tightened, unspecific reference and superfluous parentheses removed - R. J. Mathar, Oct 20 2010

cp's OEIS Frontend

A005447 Numerators of the expansion of -W_{-1}(-e^(-1 - x^2/2)) where x > 0 and W_{-1} is the Lambert W function.

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A065973 Denominators in an asymptotic expansion of Ramanujan.

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A299430 Numerators of coefficients in C(x) where: C(x)^(1/2) - S(x)^(1/2) = 1 such that C'(x)*S(x)^(1/2) = S'(x)*C(x)^(1/2) = 2*x*C(x).

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A299431 Denominators of coefficients in C(x) where: C(x)^(1/2) - S(x)^(1/2) = 1 such that C'(x)*S(x)^(1/2) = S'(x)*C(x)^(1/2) = 2*x*C(x).

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A090804 Numerators in an asymptotic expansion of Ramanujan.

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A299432 Numerators of coefficients in S(x) where: C(x)^(1/2) - S(x)^(1/2) = 1 such that C'(x)*S(x)^(1/2) = S'(x)*C(x)^(1/2) = 2*x*C(x).

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A299433 Denominators of coefficients in S(x) where: C(x)^(1/2) - S(x)^(1/2) = 1 such that C'(x)*S(x)^(1/2) = S'(x)*C(x)^(1/2) = 2*x*C(x).

A299430 Numerators of coefficients in C(x) where: C(x)^(1/2) - S(x)^(1/2) = 1 such that C'(x)S(x)^(1/2) = S'(x)C(x)^(1/2) = 2xC(x).

A299431 Denominators of coefficients in C(x) where: C(x)^(1/2) - S(x)^(1/2) = 1 such that C'(x)S(x)^(1/2) = S'(x)C(x)^(1/2) = 2xC(x).

A299432 Numerators of coefficients in S(x) where: C(x)^(1/2) - S(x)^(1/2) = 1 such that C'(x)S(x)^(1/2) = S'(x)C(x)^(1/2) = 2xC(x).

A299433 Denominators of coefficients in S(x) where: C(x)^(1/2) - S(x)^(1/2) = 1 such that C'(x)S(x)^(1/2) = S'(x)C(x)^(1/2) = 2xC(x).