A002019 -id:A002019 - cp's OEIS frontend

A000246 Number of permutations in the symmetric group S_n that have odd order.

1, 1, 1, 3, 9, 45, 225, 1575, 11025, 99225, 893025, 9823275, 108056025, 1404728325, 18261468225, 273922023375, 4108830350625, 69850115960625, 1187451971330625, 22561587455281875, 428670161650355625, 9002073394657468125, 189043541287806830625

Offset: 0

N. J. A. Sloane

Michael Reid (mreid(AT)math.umass.edu) points out that the e.g.f. for the number of permutations of odd order can be obtained from the cycle index for S_n, F(Y; X1, X2, X3, ... ) := e^(X1 Y + X2 Y^2/2 + X3 Y^3/3 + ... ) and is F(Y, 1, 0, 1, 0, 1, 0, ... ) = sqrt((1 + Y)/(1 - Y)).
a(n) appears to be the number of permutations on [n] whose up-down signature has nonnegative partial sums. For example, the up-down signature of (2,4,5,1,3) is (+1,+1,-1,+1) with nonnegative partial sums 1,2,1,2 and a(3)=3 counts (1,2,3), (1,3,2), (2,3,1). - David Callan, Jul 14 2006
This conjecture has been confirmed, see Bernardi, Duplantier, Nadeau link.
a(n) is the number of permutations of [n] for which all left-to-right minima occur in odd locations in the permutation. For example, a(3)=3 counts 123, 132, 231. Proof: For such a permutation of length 2n, you can append 1,2,..., or 2n+1 (2n+1 choices) and increase by 1 the original entries that weakly exceed the appended entry. This gives all such permutations of length 2n+1. But if the original length is 2n-1, you cannot append 1 (for then 1 would be a left-to-right min in an even location) so you can only append 2,3,..., or 2n (2n-1 choices). This count matches the given recurrence relation a(2n)=(2n-1)a(2n-1), a(2n+1)=(2n+1)a(2n). - David Callan, Jul 22 2008
a(n) is the n-th derivative of exp(arctanh(x)) at x = 0. - Michel Lagneau, May 11 2010
a(n) is the absolute value of the Moebius number of the odd partition poset on a set of n+1 points, where the odd partition poset is defined to be the subposet of the partition poset consisting of only partitions using odd part size (as well as the maximum element for n even). - Kenneth M Monks, May 06 2012
Number of permutations in S_n in which all cycles have odd length. - Michael Somos, Mar 17 2019
a(n) is the number of unranked labeled binary trees compatible with the binary labeled perfect phylogeny that, among possible two-leaf binary labeled perfect phylogenies for a sample of size n+2, is compatible with the smallest number of unranked labeled binary trees. - Noah A Rosenberg, Jan 16 2025

			For the Wallis numerators, denominators and partial products see A001900. - _Wolfdieter Lang_, Dec 06 2017

H.-D. Ebbinghaus et al., Numbers, Springer, 1990, p. 146.
J. Riordan, An Introduction to Combinatorial Analysis, Wiley, 1958, p. 87.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Alois P. Heinz, Table of n, a(n) for n = 0..450 (first 101 terms from T. D. Noe)
Ron M. Adin, Pál Hegedűs, and Yuval Roichman, Descent set distribution for permutations with cycles of only odd or only even lengths, arXiv:2502.03507 [math.CO], 2025. See p. 2.
Joel Barnes, Conformal welding of uniform random trees, Ph. D. Dissertation, Univ. Washington, 2014.
Olivier Bernardi, Bertrand Duplantier and Philippe Nadeau, A Bijection Between Well-Labelled Positive Paths and Matchings, Séminaire Lotharingien de Combinatoire (2010), volume 63, Article B63e.
William Y. C. Chen, Breaking Cycles, the Odd Versus the Even, 2023.
A. Edelman and M. La Croix, The Singular Values of the GUE (Less is More), arXiv preprint arXiv:1410.7065 [math.PR], 2014-2015. See Table 1.
Steven Finch, Rounds, Color, Parity, Squares, arXiv:2111.14487 [math.CO], 2021.
A. Ghitza and A. McAndrew, Experimental evidence for Maeda's conjecture on modular forms, arXiv preprint arXiv:1207.3480 [math.NT], 2012.
Y. Cha, Closed form solutions of difference equations (2011) PhD Thesis, Florida State University, page 24
Dmitry Kruchinin, Integer properties of a composition of exponential generating functions, arXiv:1211.2100 [math.NT], 2012.
Zhicong Lin, David G.L. Wang, and Tongyuan Zhao, A decomposition of ballot permutations, pattern avoidance and Gessel walks, arXiv:2103.04599 [math.CO], 2021.
Kenneth M. Monks, An Elementary Proof of the Explicit Formula for the Möbius Number of the Odd Partition Poset, J. Int. Seq., Vol. 21 (2018), Article 18.9.6.
Julia A. Palacios, Anand Bhaskar, Filippo Disanto, and Noah A. Rosenberg, Enumeration of binary trees compatible with a perfect phylogeny, J. Math. Biol. 84 (2022), 54.
Qingchun Ren, Ordered Partitions and Drawings of Rooted Plane Trees, arXiv preprint arXiv:1301.6327 [math.CO], 2013. See Lemma 15.
Marko Riedel, et al. From combinatorial class to recurrence to closed form, Mathematics Stack Exchange.
Jonathan Sondow, A faster product for Pi and a new integral for ln(Pi/2), arXiv:math/0401406 [math.NT], 2004.
Jonathan Sondow, A faster product for Pi and a new integral for ln(Pi/2), Amer. Math. Monthly 112 (2005), 729-734 and 113 (2006), 670.
Sam Spiro, Ballot Permutations, Odd Order Permutations, and a New Permutation Statistic, arXiv:1810.00993 [math.CO], 2018.
Paveł Szabłowski, Beta distributions whose moment sequences are related to integer sequences listed in the OEIS, Contrib. Disc. Math. (2024) Vol. 19, No. 4, 85-109. See p. 108.
Allen Wang, Permutations with Up-Down Signatures of Nonnegative Partial Sums, MIT PRIMES Conference (2018).
David G. L. Wang and T. Zhao, The peak and descent statistics over ballot permutations, arXiv:2009.05973 [math.CO], 2020.
Index entries for sequences related to groups

Cf. A001900, A059838, A002867.
Bisections are A001818 and A079484.
Row sums of unsigned triangle A049218 and of A111594, A262125.
Main diagonal of A262124.
Cf. A002019.

a000246 n = a000246_list !! n
a000246_list = 1 : 1 : zipWith (+)
   (tail a000246_list) (zipWith (*) a000246_list a002378_list)
-- Reinhard Zumkeller, Feb 27 2012

I:=[1,1]; [n le 2 select I[n] else Self(n-1)+(n^2-5*n+6)*Self(n-2): n in [1..30]]; // Vincenzo Librandi, May 02 2015

a:= proc(n) option remember; `if`(n<2, 1,
      a(n-1) +(n-1)*(n-2)*a(n-2))
    end:
seq(a(n), n=0..25);  # Alois P. Heinz, May 14 2018

a[n_] := a[n] = a[n-1]*(n+Mod[n, 2]-1); a[0] = 1; Table[a[n], {n, 0, 20}] (* Jean-François Alcover, Nov 21 2011, after Pari *)
a[n_] := a[n] = (n-2)*(n-3)*a[n-2] + a[n-1]; a[0] := 0; a[1] := 1; Table[a[i], {i, 0, 20}] (* or *)  RecurrenceTable[{a[0]==0, a[1]==1, a[n]==(n-2)*(n-3)a[n-2]+a[n-1]}, a, {n, 20}] (* G. C. Greubel, May 01 2015 *)
CoefficientList[Series[Sqrt[(1+x)/(1-x)], {x, 0, 20}], x]*Table[k!, {k, 0, 20}] (* Stefano Spezia, Oct 07 2018 *)

PARI
```
a(n)=if(n<1,!n,a(n-1)*(n+n%2-1))
```

Vec( serlaplace( sqrt( (1+x)/(1-x) + O(x^55) ) ) )

a(n)=prod(k=3,n,k+k%2-1) \\ Charles R Greathouse IV, May 01 2015

a(n)=(n!/(n\2)!>>(n\2))^2/if(n%2,n,1) \\ Charles R Greathouse IV, May 01 2015

E.g.f.: sqrt(1-x^2)/(1-x) = sqrt((1+x)/(1-x)).
a(2*k) = (2*k-1)*a(2*k-1), a(2*k+1) = (2*k+1)*a(2*k), for k >= 0, with a(0) = 1.
Let b(1)=0, b(2)=1, b(k+2)=b(k+1)/k + b(k); then a(n+1) = n!*b(n+2). - Benoit Cloitre, Sep 03 2002
a(n) = Sum_{k=0..floor((n-1)/2)} (2k)! * C(n-1, 2k) * a(n-2k-1) for n > 0. - Noam Katz (noamkj(AT)hotmail.com), Feb 27 2001
Also successive denominators of Wallis's approximation to Pi/2 (unreduced): 1/1 *  2/1 * 2/3 * 4/3 * 4/5 * 6/5 * 6/7 * .., for n >= 1.
D-finite with recurrence: a(n) = a(n-1) + (n-1)*(n-2)*a(n-2). - Benoit Cloitre, Aug 30 2003
a(n) is asymptotic to (n-1)!*sqrt(2*n/Pi). - Benoit Cloitre, Jan 19 2004
a(n) = n! * binomial(n-1, floor((n-1)/2)) / 2^(n-1), n > 0. - Ralf Stephan, Mar 22 2004
E.g.f.: e^atanh(x), a(n) = n!*Sum_{m=1..n} Sum_{k=m..n} 2^(k-m)*Stirling1(k,m) *binomial(n-1,k-1)/k!, n > 0, a(0)=1. - Vladimir Kruchinin, Dec 12 2011
G.f.: G(0) where G(k) = 1 + x*(4*k-1)/((2*k+1)*(x-1) - x*(x-1)*(2*k+1)*(4*k+1)/(x*(4*k+1) + 2*(x-1)*(k+1)/G(k+1))); (continued fraction, 3rd kind, 3-step). - Sergei N. Gladkovskii, Jul 24 2012
G.f.: 1 + x*(G(0) - 1)/(x-1) where G(k) = 1 - (2*k+1)/(1-x/(x - 1/(1 - (2*k+1)/(1-x/(x - 1/G(k+1) ))))); (continued fraction). - Sergei N. Gladkovskii, Jan 15 2013
G.f.: G(0), where G(k) = 1 + x*(2*k+1)/(1 - x*(2*k+1)/(x*(2*k+1) + 1/G(k+1))); (continued fraction). - Sergei N. Gladkovskii, Jun 07 2013
For n >= 1, a(2*n) = (2*n-1)!!^2, a(2*n+1) = (2*n+1)*(2*n-1)!!^2. - Vladimir Shevelev, Dec 01 2013
E.g.f.: arcsin(x) - sqrt(1-x^2) + 1 for a(0) = 0, a(1) = a(2) = a(3) = 1. - G. C. Greubel, May 01 2015
Sum_{n>1} 1/a(n) = (L_0(1) + L_1(1))*Pi/2, where L is the modified Struve function. - Peter McNair, Mar 11 2022
From Peter Bala, Mar 29 2024: (Start)
a(n) =  n! * Sum_{k = 0..n} (-1)^(n+k)*binomial(1/2, k)*binomial(-1/2, n-k).
a(n) = (1/4^n) * (2*n)!/n! * hypergeom([-1/2, -n], [1/2 - n], -1).
a(n) = n!/2^n * A063886(n). (End)

A006228 Expansion of e.g.f. exp(arcsin(x)).

Original entry on oeis.org

1, 1, 1, 2, 5, 20, 85, 520, 3145, 26000, 204425, 2132000, 20646925, 260104000, 2993804125, 44217680000, 589779412625, 9993195680000, 151573309044625, 2898026747200000, 49261325439503125, 1049085682486400000, 19753791501240753125, 463695871658988800000

Offset: 0

N. J. A. Sloane, Jeffrey Shallit

nonn

L. B. W. Jolley, Summation of Series. 2nd ed., Dover, NY, 1961, p. 150.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

T. D. Noe, Table of n, a(n) for n = 0..100
H. S. Uhler, On the numerical value of i^i, Amer. Math. Monthly, 28 (1921), 114-116.
Kruchinin Vladimir Victorovich, Composition of ordinary generating functions, arXiv:1009.2565 [math.CO], 2010.

Bisections are expansions of sin(arcsinh(x)) and cos(arcsinh(x)).
Bisections are A101927 and A101928.
Row sums of A385343.
Cf. A002019.
Cf. A166741, A166748. - Jaume Oliver Lafont, Oct 24 2009

a:= n-> n!*coeff(series(exp(arcsin(x)), x, n+1), x, n):
seq(a(n), n=0..25);  # Alois P. Heinz, Aug 17 2018

Distribute[ CoefficientList[ Series[ E^ArcSin[x], {x, 0, 21}], x] * Table[ n!, {n, 0, 21}]] (* Robert G. Wilson v, Feb 10 2004 *)
With[{nn=30},CoefficientList[Series[Exp[ArcSin[x]],{x,0,nn}],x]Range[0,nn]!] (* Harvey P. Dale, Feb 26 2013 *)
Table[FullSimplify[2^(n-2) * (Exp[Pi/2]-(-1)^n*Exp[-Pi/2]) * Gamma[(n-I)/2] * Gamma[(n+I)/2] / Pi], {n, 0, 20}] (* Vaclav Kotesovec, Nov 06 2014 *)

a(n):=(n-1)!*sum((if n=m then 1 else if oddp(n-m) then 0 else sum((-1)^k*(sum(binomial(k,j)*2^(1-j)*sum((-1)^((n-m)/2-i)*binomial(j,i)*(j-2*i)^(n-m+j)/(n-m+j)!,i,0,floor(j/2))*(-1)^(k-j),j,1,k))*binomial(k+n-1,n-1),k,1,n-m))/(m-1)!,m,1,n); /* Vladimir Kruchinin, Sep 12 2010 */

i even: a_i = Product_{j=1..i/2-1} 1 + 4j^2, i odd: a_i = Product_{j=1..(i-1)/2} 2 + 4j(j-1). - Cris Moore (moore(AT)santafe.edu), Jan 31 2001
a(0)=1, a(1)=1, a(n) = (1+(n-2)^2)*a(n-2) for n >= 2. Jaume Oliver Lafont, Oct 24 2009
a(n) = (n-1)!*sum((if n=m then 1 else if oddp(n-m) then 0 else sum((-1)^k*(sum(C(k,j)*2^(1-j)*sum((-1)^((n-m)/2-i)*C(j,i)*(j-2*i)^(n-m+j)/(n-m+j)!, i=0..floor(j/2))*(-1)^(k-j), j=1..k))*C(k+n-1,n-1), k=1..n-m))/(m-1)!, m=1..n), n>0. - Vladimir Kruchinin, Sep 12 2010
E.g.f.: exp(arcsin(x))=1+2z/(H(0)-z); H(k)=4k+2+z^2*(4k^2+8k+5)/H(k+1), where z=x/((1-x^2)^1/2); (continued fraction). - Sergei N. Gladkovskii, Nov 20 2011
a(n) ~ (exp(Pi/2)-(-1)^n*exp(-Pi/2)) * n^(n-1) / exp(n). - Vaclav Kotesovec, Oct 23 2013
a(n) = 2^(n-2) * (exp(Pi/2)-(-1)^n*exp(-Pi/2)) * GAMMA((n-I)/2) * GAMMA((n+I)/2) / Pi. - Vaclav Kotesovec, Nov 06 2014

More terms from Christian G. Bower

A049218 Triangle T(n,k) of arctangent numbers: expansion of arctan(x)^n/n!.

Original entry on oeis.org

1, 0, 1, -2, 0, 1, 0, -8, 0, 1, 24, 0, -20, 0, 1, 0, 184, 0, -40, 0, 1, -720, 0, 784, 0, -70, 0, 1, 0, -8448, 0, 2464, 0, -112, 0, 1, 40320, 0, -52352, 0, 6384, 0, -168, 0, 1, 0, 648576, 0, -229760, 0, 14448, 0, -240, 0, 1, -3628800, 0, 5360256, 0, -804320, 0, 29568, 0, -330, 0, 1

Offset: 1

N. J. A. Sloane

|T(n,k)| gives the sum of the M_2 multinomial numbers (A036039) for those partitions of n with exactly k odd parts. E.g.: |T(6,2)| = 144 + 40 = 184 from the partitions of 6 with exactly two odd parts, namely (1,5) and (3,3), with M_2 numbers 144 and 40. Proof via the general Jabotinsky triangle formula for |T(n,k)| using partitions of n into k parts and their M_3 numbers (A036040). Then with the special e.g.f. of the (unsigned) k=1 column, f(x):= arctanh(x), only odd parts survive and the M_3 numbers are changed into the M_2 numbers. For the Knuth reference on Jabotinsky triangles see A039692. - Wolfdieter Lang, Feb 24 2005 [The first two sentences have been corrected thanks to the comment by José H. Nieto S. given below. - Wolfdieter Lang, Jan 16 2012]
|T(n,k)| gives the number of permutations of {1,2,...,n} (degree n permutations) with the number of odd cycles equal to k. E.g.: |T(5,3)|= 20 from the 20 degree 5 permutations with cycle structure (.)(.)(...). Proof: Use the cycle index polynomial for the symmetric group S_n (see the M_2 array A036039 or A102189) together with the partition interpretation of |T(n,k)| given above. - Wolfdieter Lang, Feb 24 2005 [See the following José H. Nieto S. correction. - Wolfdieter Lang, Jan 16 2012]
The first sentence of the above comment is inexact, it should be "|T(n,k)| gives the number of degree n permutations which decompose into exactly k odd cycles". The number of degree n permutations with k odd cycles (and, possibly, other cycles of even length) is given by A060524. - José H. Nieto S., Jan 15 2012
The unsigned triangle with e.g.f. exp(x*arctanh(z)) is the associated Jabotinsky type triangle for the Sheffer type triangle A060524. See the comments there. - Wolfdieter Lang, Feb 24 2005
Also the Bell transform of the sequence (-1)^(n/2)*A005359(n) without column 0. For the definition of the Bell transform see A264428. - Peter Luschny, Jan 28 2016

			Triangle begins:
   1;
   0,   1;
  -2,   0,   1;
   0,  -8,   0,   1;
  24,   0, -20,   0,   1;
   0, 184,   0, -40,   0,   1;
  ...
O.g.f. for fifth subdiagonal: (24*t+16*t^2)/(1-t)^7 = 24*t + 184*t^2 + 784*t^3 + 2404*t^4 + ....

L. Comtet, Advanced Combinatorics, Reidel, 1974, p. 260.

Peter Bala, Diagonals of triangles with generating function exp(t*F(x)).
Steven Finch, Rounds, Color, Parity, Squares, arXiv:2111.14487 [math.CO], 2021.
Kruchinin Vladimir Victorovich, Composition of ordinary generating functions, arXiv:1009.2565 [math.CO], 2010.
Eric Weisstein's World of Mathematics, Mittag-Leffler Polynomial

Essentially same as A008309, which is the main entry for this sequence.

Row sums (unsigned) give A000246(n); signed row sums give A002019(n), n>=1. A137513.

A049218 := proc(n,k)(-1)^((3*n+k)/2) *add(2^(j-k)*n!/j! *stirling1(j,k) *binomial(n-1,j-1),j=k..n) ; end proc: # R. J. Mathar, Feb 14 2011
# The function BellMatrix is defined in A264428.
# Adds (1,0,0,0, ..) as column 0.
BellMatrix(n -> `if`(n::odd, 0, (-1)^(n/2)*n!), 10); # Peter Luschny, Jan 28 2016

t[n_, k_] := (-1)^((3n+k)/2)*Sum[ 2^(j-k)*n!/j!*StirlingS1[j, k]*Binomial[n-1, j-1], {j, k, n}]; Flatten[ Table[ t[n, k], {n, 1, 11}, {k, 1, n}]] (* Jean-François Alcover, Dec 06 2011, after Vladimir Kruchinin *)
BellMatrix[f_Function, len_] := With[{t = Array[f, len, 0]}, Table[BellY[n, k, t], {n, 0, len-1}, {k, 0, len-1}]];
rows = 12;
M = BellMatrix[If[OddQ[#], 0, (-1)^(#/2)*#!]&, rows];
Table[M[[n, k]], {n, 2, rows}, {k, 2, n}] // Flatten (* Jean-François Alcover, Jun 23 2018, after Peter Luschny *)

T(n,k)=polcoeff(serlaplace(atan(x)^k/k!), n)

E.g.f.: arctan(x)^k/k! = Sum_{n>=0} T(n, k) x^n/n!.
T(n,k) = ((-1)^((3*n+k)/2)*n!/2^k)*Sum_{i=k..n} 2^i*binomial(n-1,i-1)*Stirling1(i,k)/i!. - Vladimir Kruchinin, Feb 11 2011
E.g.f.: exp(t*arctan(x)) = 1 + t*x + t^2*x^2/2! + t*(t^2-2)*x^3/3! + .... The unsigned row polynomials are the Mittag-Leffler polynomials M(n,t/2). See A137513. The compositional inverse (with respect to x) (x-t/2*log((1+x)/(1-x)))^(-1) = x/(1-t) + 2*t/(1-t)^4*x^3/3!+ (24*t+16*t^2)/(1-t)^7*x^5/5! + .... The rational functions in t generate the (unsigned) diagonals of the table. See the Bala link. - Peter Bala, Dec 04 2011

Additional comments from Michael Somos

A331617 E.g.f.: exp(1 / (1 - arctan(x)) - 1).

Original entry on oeis.org

1, 1, 3, 11, 49, 265, 1683, 12035, 95169, 832337, 7998467, 83033403, 922112305, 10978263257, 139956480467, 1889161216179, 26798589518593, 401123509624737, 6346168059440515, 105040097140558699, 1805102151607613361, 32421358229074354601

Offset: 0

Ilya Gutkovskiy, Jan 22 2020

sign

a(53) is negative. - Vaclav Kotesovec, Jan 26 2020

Vaclav Kotesovec, Table of n, a(n) for n = 0..400

Cf. A002019, A110708, A191700, A331610, A331615, A331616, A331618.

nmax = 21; CoefficientList[Series[Exp[1/(1 - ArcTan[x]) - 1], {x, 0, nmax}], x] Range[0, nmax]!
A191700[0] = 1; A191700[n_] := A191700[n] = Sum[Binomial[n, k] If[OddQ[k], (-1)^Boole[IntegerQ[(k + 1)/4]] (k - 1)!, 0] A191700[n - k], {k, 1, n}]; a[0] = 1; a[n_] := a[n] = Sum[Binomial[n - 1, k - 1] A191700[k] a[n - k], {k, 1, n}]; Table[a[n], {n, 0, 21}]

seq(n)={Vec(serlaplace(exp(1/(1 - atan(x + O(x*x^n))) - 1)))} \\ Andrew Howroyd, Jan 22 2020

a(0) = 1; a(n) = Sum_{k=1..n} binomial(n-1,k-1) * A191700(k) * a(n-k).

A102059 Expansion of e.g.f. cos(arctanh(x)), even powers only.

Original entry on oeis.org

1, -1, -7, -145, -6095, -433025, -46676375, -7108596625, -1454225641375, -384836032842625, -127950804666254375, -52219402100109700625, -25668587693366081579375, -14959038795678519196890625, -10198912212548907619042984375, -8042754039731999959020139140625

Offset: 1

Ralf Stephan, Dec 28 2004

sign

			cos(arctanh(x)) = 1 - x^2/2 - 7x^4/4! - 145x^6/6! - 6095x^8/8! - ...

Vincenzo Librandi, Table of n, a(n) for n = 1..100

Bisection of A002019.

m:=35; R:=PowerSeriesRing(Rationals(), m);  b:=Coefficients(R!(1+Cos(Argtanh(x)))); [1] cat [Factorial(n-1)*b[n]: n in [3..m by 2]]; // Vincenzo Librandi, Aug 16 2018

nmax=20; Table[(CoefficientList[Series[Cos[ArcTanh[x]],{x,0,2*nmax}],x] * Range[0,2*nmax]!)[[n]],{n,1,2*nmax,2}] (* Vaclav Kotesovec, Nov 06 2014 *)

A102058 Expansion of e.g.f. sin(arctanh(x)), odd powers only.

Original entry on oeis.org

1, 1, 5, 5, -5815, -956375, -172917875, -38579649875, -10713341611375, -3663118565923375, -1519935859717136875, -754429769289426936875, -442113820341129750734375, -302333022017412857174234375, -238762676857713027642764171875, -215766282905942334008224968671875

Offset: 1

Ralf Stephan, Dec 28 2004

sign

			sin(arctanh(x)) = x + x^3/3! + 5x^5/5! + 5x^7/7! - 5815x^9/9! - ...

Vincenzo Librandi, Table of n, a(n) for n = 1..100

Bisection of A002019.

m:=35; R:=PowerSeriesRing(Rationals(), m);  b:=Coefficients(R!(1+Sin(Argtanh(x)))); [Factorial(n-1)*b[n]: n in [2..m by 2]]; // Vincenzo Librandi, Aug 16 2018

nmax=20; Table[(CoefficientList[Series[Sin[ArcTanh[x]],{x,0,2*nmax}],x] * Range[0,2*nmax-1]!)[[n]],{n,2,2*nmax,2}] (* Vaclav Kotesovec, Nov 06 2014 *)

a(n):=(2*n-1)!*sum((-1)^(i)*sum((stirling1(k+2*i+1,2*i+1)*2^(k)* binomial(2*n-2,k+2*i))/(k+2*i+1)!,k,0,2*n-1-2*i-1),i,0,n-1); /* Vladimir Kruchinin, Dec 12 2011 */

a(n) = (2*n-1)!*Sum(i=0..n-1, (-1)^(i)*Sum(k=0..2*n-1-2*i-1, (stirling1(k+2*i+1,2*i+1)*2^(k)* binomial(2*n-2,k+2*i))/(k+2*i+1)!)). - Vladimir Kruchinin, Dec 12 2011

A168406 E.g.f.: Sum_{n>=0} arctan(2^n*x)^n/n!.

Original entry on oeis.org

1, 2, 16, 496, 63488, 32899840, 68049141760, 560546415810560, 18415229458563727360, 2416302337337071616327680, 1267360474688679165942982246400, 2658246833688954938616062542151680000

Offset: 0

Paul D. Hanna, Nov 25 2009

nonn

			E.g.f.: A(x) = 1 + 2*x + 16*x^2/2! + 496*x^3/3! + 63488*x^4/4! + ...
A(x) = 1 + arctan(2*x) + arctan(4*x)^2/2! + arctan(8*x)^3/3! + arctan(16*x)^4/4! + ... + arctan(2^n*x)^n/n! + ...
a(n) = coefficient of x^n/n! in G(x)^(2^n) where G(x) = exp(arctan(x)):
G(x) = 1 + x + x^2/2! - x^3/3! - 7*x^4/4! + 5*x^5/5! + 145*x^6/6! + ... + A002019(n)*x^n/n! + ...

Cf. A002019 (exp(arctan x)), variants: A136632, A168402, A168403, A168404, A168405, A168407, A168408.

{a(n)=n!*polcoeff(sum(k=0,n,atan(2^k*x +x*O(x^n))^k/k!),n)}

{a(n)=n!*polcoeff(exp(2^n*atan(x +x*O(x^n))),n)}

a(n) = [x^n/n!] exp(2^n*arctan(x)) for n >= 0.

A293192 a(n) = n! * [x^n] exp(n*arctan(x)).

Original entry on oeis.org

1, 1, 4, 21, 128, 745, 1440, -89075, -3031040, -76951215, -1784742400, -39056351675, -785147904000, -12815410658375, -64977212518400, 8229576066616125, 587339977981952000, 29185361934747006625, 1256364637107240960000, 48860370609512175371125, 1686271331087959982080000

Offset: 0

Ilya Gutkovskiy, Oct 02 2017

sign

Cf. A002019.

Table[n! SeriesCoefficient[Exp[n ArcTan[x]], {x, 0, n}], {n, 0, 20}]
Table[n! SeriesCoefficient[Exp[n Sum[(-1)^k x^(2 k + 1)/(2 k + 1), {k, 0, Infinity}]], {x, 0, n}], {n, 0, 20}]

A296787 Expansion of e.g.f. exp(x*arctan(x)) (even powers only).

Original entry on oeis.org

1, 2, 4, 24, -496, 36000, -3753408, 556961664, -111591202560, 29054584410624, -9541382573767680, 3858875286730168320, -1884995591107521540096, 1094305223336273239449600, -744771228363250138965196800, 587358379156469629707528929280

Offset: 0

Ilya Gutkovskiy, Dec 20 2017

sign

			exp(x*arctan(x)) = 1 + 2*x^2/2! + 4*x^4/4! + 24*x^6/6! - 496*x^8/8! + ...

Cf. A002019, A009252, A009273, A010050, A102059, A166356, A259647, A293192, A296788, A296789.

nmax = 15; Table[(CoefficientList[Series[Exp[x ArcTan[x]], {x, 0, 2 nmax}], x] Range[0, 2 nmax]!)[[n]], {n, 1, 2 nmax + 1, 2}]
nmax = 15; Table[(CoefficientList[Series[Exp[(I/2) x (Log[1 - I x] - Log[1 + I x])], {x, 0, 2 nmax}], x] Range[0, 2 nmax]!)[[n]], {n, 1, 2 nmax + 1, 2}]

a(n) = (2*n)! * [x^(2*n)] exp(x*arctan(x)).

a(n) ~ -(-1)^n * 2^(2*n-1) * n^(2*n-1) / exp(2*n). - Vaclav Kotesovec, Dec 21 2017

A012960 Expansion of e.g.f. exp(arctan(x)+log(x+1)).

Original entry on oeis.org

1, 2, 3, 2, -11, -30, 175, 1010, -6135, -60670, 374875, 5719650, -35199875, -779710750, 4687746375, 145208599250, -836951243375, -35435177514750, 191995883837875, 10975003189933250, -54688433347786875

Offset: 0

Patrick Demichel (patrick.demichel(AT)hp.com)

sign

			1+2*x+3/2!*x^2+2/3!*x^3-11/4!*x^4-30/5!*x^5...

Cf. A002019.

With[{nn=20},CoefficientList[Series[Exp[ArcTan[x]+Log[x+1]],{x,0,nn}],x] Range[0,nn]!] (* Harvey P. Dale, Jul 25 2013 *)

my(x='x+O('x^30)); Vec(serlaplace(exp(atan(x)+log(x+1)))) \\ Michel Marcus, May 10 2020

a(n) = (n*A002019(n)-A002019(n+1))/(n-1) for n > 1. [Mark van Hoeij, Jul 02 2010]

Definition clarified by Harvey P. Dale, Jul 25 2013

cp's OEIS Frontend

A000246 Number of permutations in the symmetric group S_n that have odd order.

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