A001570 -id:A001570 - cp's OEIS frontend

A242394 Number of equilateral triangles (sides length = 1) that intersect the circumference of a circle of radius n centered at (0,0).

6, 18, 30, 42, 54, 66, 66, 102, 114, 126, 138, 150, 150, 162, 198, 210, 222, 234, 222, 270, 258, 294, 306, 318, 330, 330, 366, 354, 390, 402, 390, 426, 450, 462, 450, 486, 474, 486, 510, 546, 558, 546, 558, 594, 606, 630, 642, 654, 618, 678, 690, 690, 726, 738, 750, 738, 750

Offset: 1

Kival Ngaokrajang, May 13 2014

nonn

For all n, there are at least 6 points where the transit of circumference occurs exactly at the corners. The rare case is when the transit occurs at 2 corners of a triangle, i.e., at n = 1, 13, 181, 35113, ... , (A001570(n)). The pattern repeats itself at every Pi/3 sector along the circumference. The triangle count per half sector by rows can be arranged as an irregular triangle as shown in the illustration. The rows count (A242396) is equal to the case centered at (1/2,0), A242395.

Kival Ngaokrajang, Illustration of initial terms
Kival Ngaokrajang, Illustration for rare cases

Cf. A001570, A242118.

A340295 a(n) = 4^(2n^2) Product_{1<=j,k<=n} (1 - sin(jPi/(2n+1))^2 * cos(kPi/(2n+1))^2).

Original entry on oeis.org

1, 13, 18281, 2732887529, 43384923739812577, 73125714588602035608260981, 13085551252412040683513520733767180041, 248596840858215958581954513797323868183183928594833

Offset: 0

Seiichi Manyama, Jan 03 2021

nonn

a(n)/A001570(n+1) is an integer.

Wikipedia, Chebyshev polynomials
Wikipedia, Resultant

Cf. A001570, A334089, A340166, A340291, A340292.

Table[Resultant[ChebyshevT[4*n+2, x/2], ChebyshevT[4*n+2, I*x/2], x]^(1/4) / 2^n, {n, 0, 10}] (* Vaclav Kotesovec, Jan 04 2021 *)

default(realprecision, 120);
{a(n) = round(4^(2*n^2)*prod(j=1, n, prod(k=1, n, 1-(sin(j*Pi/(2*n+1))*cos(k*Pi/(2*n+1)))^2)))}

{a(n) = sqrtint(sqrtint(polresultant(polchebyshev(4*n+2, 1, x/2), polchebyshev(4*n+2, 1, I*x/2))))/2^n}

a(n) = A334089(2*n+1).

a(n) ~ exp(2*G*(2*n+1)^2/Pi) / 2^(3*n + 7/8), where G is Catalan's constant A006752. - Vaclav Kotesovec, Jan 04 2021

A006244 Hexagonal numbers (A000384) which are also centered hexagonal numbers (A003215).

Original entry on oeis.org

1, 91, 8911, 873181, 85562821, 8384283271, 821574197731, 80505887094361, 7888755361049641, 773017519495770451, 75747828155224454551, 7422514141692500775541, 727330638057709851548461, 71270980015513872950973631, 6983828710882301839343867371, 684343942686450066382748028721

Offset: 1

N. J. A. Sloane, Jeffrey Shallit

Equivalently, triangular hex numbers.

			a(1)=91 because 91 is the sixth centered hexagonal number and the seventh hexagonal number.

M. Gardner, Time Travel and Other Mathematical Bewilderments. Freeman, NY, 1988, p. 19.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Jon E. Schoenfield, Table of n, a(n) for n = 1..500
Simon Plouffe, Approximations de séries génératrices et quelques conjectures, Dissertation, Université du Québec à Montréal, 1992; arXiv:0911.4975 [math.NT], 2009.
Simon Plouffe, 1031 Generating Functions, Appendix to Thesis, Montreal, 1992
S. C. Schlicker, Numbers Simultaneously Polygonal and Centered Polygonal, Mathematics Magazine, Vol. 84, No. 5, December 2011, pp. 339-350.
Eric Weisstein's World of Mathematics, Hex Number
Index entries for linear recurrences with constant coefficients, signature (99,-99,1).

Cf. A001570, A001921, A000384, A003215, A253175.

CP := n -> 1+1/2*6*(n^2-n): N:=10: u:=5: v:=1: x:=6: y:=1: k_pcp:=[1]: for i from 1 to N do tempx:=x; tempy:=y; x:=tempx*u+24*tempy*v: y:=tempx*v+tempy*u: s:=(y+1)/2: k_pcp:=[op(k_pcp),CP(s)]: end do: k_pcp; # Steven Schlicker, Apr 24 2007
A006244:=-(1-8*z+z**2)/(z-1)/(z**2-98*z+1); # Conjectured (correctly) by Simon Plouffe in his 1992 dissertation.
a := n -> (Matrix([[91,1,1]]). Matrix([[99,1,0],[ -99,0,1],[1,0,0]])^n)[1,3]; seq (a(n), n=1..20); # Alois P. Heinz, Aug 14 2008

CoefficientList[Series[(1 - 8*x + x^2)/(1 - 99*x + 99*x^2 - x^3), {x, 0, 20}], x] (* Jean-François Alcover, Feb 26 2015 *)

Vec(-x*(x^2-8*x+1)/((x-1)*(x^2-98*x+1)) + O(x^100)) \\ Colin Barker, Jan 08 2015

From Richard Choulet, Sep 19 2007: (Start)
We must solve 2*r^2-r=3*p^2-3*p+1, which gives X^2=6*Y^2+3 with X=4*r-1 and Y=2*p-1. We obtain at the same time the following sequences:
X is given by 3, 27, 267, ... sequence for which a(n+2)=10*a(n+1)-a(n) and a(n+1)=5*a(n)+2*(6a(n)^2-18)^0.5
Y is given by 1, 11, 109, ... sequence for which a(n+2)=10*a(n+1)-a(n) and a(n+1)=5*a(n)+2*(6a(n)^2+3)^0.5
p is given by 1, 6, 55, 540, ... sequence for which a(n+2)=10*a(n+1)-a(n)-4 and a(n+1)=5*a(n)-2+(24*a(n)^2-24*a(n)+9)^0.5
r is given by 1, 7, 67, 661, ... sequence for which a(n+2)=10*a(n+1)-a(n)-2 and a(n+1)=5*a(n)-1+(24*a(n)^2-12*a(n)-3)^0.5
a(n+2) = 98*a(n+1)-a(n)-6, a(n+1)=49*a(n)-3+5*(96*a(n)^2-12*a(n)-3)^0.5.
G.f.: z*(1-8*z+z^2)/((1-z)*(1-98*z+z^2)). (End)
Define x(n) + y(n)*sqrt(24) = (6+sqrt(24))*(5+sqrt(24))^n, s(n) = (y(n)+1)/2; then a(n) = (1/2)*(2+6*(s(n)^2-s(n))). - Steven Schlicker, Apr 24 2007
a(n) = (A007667(n+1)-1)/4. - Ralf Stephan, Mar 03 2004
a(n) = 99*a(n-1)-99*a(n-2)+a(n-3). - Colin Barker, Jan 08 2015

Edited by N. J. A. Sloane, Sep 25 2007
More terms from Alois P. Heinz, Aug 14 2008
More terms from Jon E. Schoenfield, Dec 26 2008

A117808 Primes of the form ((2 + sqrt(3))^(2k+1) + (2 - sqrt(3))^(2k+1))/4.

Original entry on oeis.org

3, 13, 181, 2521, 489061, 6811741, 1321442641, 18405321661, 381765135195632792959100810331957408101589361

Offset: 1

Roger L. Bagula, Apr 29 2006

nonn

Primes in A001570. - Joerg Arndt, Dec 30 2023
Primes among absolute values of A108946.
Also the Cosgrave-Dilcher primes that are a subset of the nontrivial cyclotomic lambda invariant for Q(sqrt{-3}) (or a subset of the 1-exceptional primes for M=3). - Christopher M. Stokes, Aug 04 2022

J. B. Cosgrave and K. Dilcher, The multiplicative orders of certain Gauss factorials, Intl. J. Number Theory 7 (1) (2011) 145-171.
John B. Cosgrave and Karl Dilcher, The multiplicative orders of certain Gauss factorials II, Funct. Approx. Comment. Math. Volume 54, Number 1 (2016), 73-93.
Christopher Stokes, On Gauss factorials and their application to Iwasawa theory for imaginary quadratic fields, arXiv:2207.07804 [math.NT], 2022.
Eric Weisstein's World of Mathematics, Prime-Generating Polynomial

Cf. A108946.

b(n)=my(w=quadgen(12)); ((w+2)^n+(2-w)^n)\4
for(n=2,800, if(isprime(p=b(n)), print1(p", "))) \\ Charles R Greathouse IV, Aug 22 2022

Definition and terms corrected by N. J. A. Sloane, May 21 2010

Edited by Joerg Arndt, Dec 30 2023

A120744 Least k>0 such that a centered polygonal number nk(k+1)/2+1 is a perfect square; or -1 if no such number exists.

Original entry on oeis.org

2, -1, 1, 3, 2, 7, 15, 1, 16, 8, 14, 4, 5, 15, 1, 2, 5, -1, 6, 3, 2, 39, 6, 1, 21, 7, 110, 3, 15, 7, 15, -1, 2, 8, 1, 4, 989, 8, 14, 2, 45, 15, 9, 4, 5, 335, 9, 1, 29, -1, 30, 15, 10, 415, 6, 2, 10, 32, 54, 3, 77, 55, 1, 5, 2, 7, 47750, 11, 15, 23, 47, -1, 48, 24, 16, 12, 5, 8, 2639, 1, 6720, 704, 38, 4, 2, 39, 505, 3, 13, 56, 9, 20, 13, 1631, 41

Offset: 1

Alexander Adamchuk, Apr 26 2007

sign

			a(5) = 2 because A129556(2) = 2>1 and A129556(1) = 0<1.

Max Alekseyev, Table of n, a(n) for n = 1..10000
Eric Weisstein's World of Mathematics, Centered Polygonal Number

Cf. A001542, A006451, A001921, A129444, A001570, A001652, A129556, A053606, A105038, A105040, A053141, A061278.

a(n) = -1 for n in A166259.

a(n) = 1 for n = k^2-1.

Edited and b-file provided by Max Alekseyev, Jan 20 2010

A088587 Difference between the two segments of edge d = A089025(n) partitioned by cevian c = A088513(n).

Original entry on oeis.org

2, 1, 11, 13, 26, 22, 23, 47, 13, 46, 11, 74, 61, 2, 37, 71, 107, 97, 23, 22, 118, 59, 146, 37, 143, 1, 109, 166, 191, 73, 83, 193, 26, 131, 94, 157, 11, 122, 239, 253, 59, 299, 73, 142, 286, 167, 169, 227, 362, 46, 121, 83, 277, 358, 218

Offset: 1

Lekraj Beedassy, Nov 20 2003

nonn

a(n) is minimal, i.e., equals 1 for values c = A001570(n), d = A028230(n).

Cf. A088514.

findPrimIntEquiSectionDiff[maxC_] :=
Reap[Do[Do[
     With[{cevian = Abs[c E^((2 \[Pi] I)/6) - a]},
      If[FractionalPart[cevian] == 0 && GCD[a, c] == 1,
       Sow[c - 2 a]; Break[]]], {a, Floor[c/2],
      1, -1}], {c, maxC}]][[2, 1]]
(* Andrew Turner, Aug 04 2017 *)

a(n) = sqrt(4c^2 - 3d^2).

a(35)=94 and a(36)=157 order reversed by Andrew Turner, Aug 04 2017

A097947 Expansion of g.f. (2+7x+2x^2)/((x^2-1)(1+4x+x^2)).

Original entry on oeis.org

-2, 1, -6, 16, -62, 225, -842, 3136, -11706, 43681, -163022, 608400, -2270582, 8473921, -31625106, 118026496, -440480882, 1643897025, -6135107222, 22896531856, -85451020206, 318907548961, -1190179175642, 4441809153600, -16577057438762, 61866420601441, -230888624967006

Offset: 0

N. J. A. Sloane, following a suggestion of Creighton Dement, Sep 06 2004

One of 4 related sequences. This is the sequence "les(n)". "jes(n)" = [1, -4, 15, -56, ...] is (-1)^(n+1)*A001353(n+1), "tes(n)" is A097948 and "ves(n)" is A099949.

Index entries for linear recurrences with constant coefficients, signature (-4,0,4,1).

Cf. A001353, A097948, A097949.

LinearRecurrence[{-4, 0, 4, 1}, {-2, 1, -6, 16}, 27] (* Robert P. P. McKone, Aug 25 2023 *)

Properties (from Creighton Dement, Sep 06 2004):
I: jes(n) + les(n) + tes(n) = ves(n)
II: All of the following are perfect squares: {les(2n+1); tes(2n+1); ves(2n+1); ves(2n+1) - jes(2n+1) - 1 = les(2n+1) + tes(2n+1) - 1; 3*les(2n+1) + 1 = 3*jes(n)^2 + 1}.
III: les(2n+1) divides ves(2n+1) - jes(2n+1) - 1 = les(2n+1) + tes(2n+1) - 1
IV: (jes(n))^2 = les(2n+1)
V: tes(2n) = A001570(n), sqrt( tes(2n+1) ) = A001075(n)
VI: sqrt( ves(2n+1) ) = A001835(n)
VII: sqrt( les(2n+1) ) = A001353(n)
VIII: les(n) + tes(n) = ves(2+n) + jes(n)
IX: lim n |jes(n+1)/jes(n)| = lim n |les(n+1)/les(n)| = lim n |tes(n+1)/tes(n)| = lim n |ves(n+1)/ves(n)| = 2 + sqrt(3)
Comment from Roland Bacher, Sep 07 2004: These 4 sequences satisfy jes(n+1)=-4*jes(n)-jes(n-1), les(n+1)=les(n-1)+jes(n), ves(n+1)=les(n-1)-jes(n-1)+tes(n-1), tes(n+1)=les(n-1)+3*jes(n), plus initial conditions for n=0, 1.
12*a(n) = -11 -9*(-1)^n -2*(-1)^n*A001075(n+1). - R. J. Mathar, May 21 2019
From Eric Simon Jacob, Aug 26 2023: (Start)
a(n) = ( ( sqrt(3) - 2 )^(n+1) + ( -sqrt(3) - 2 )^(n+1) + 9*(-1)^(n+1) - 11 )/12.
a(n) = ( 2*cosh( (n+1)*log(sqrt(3) - 2) ) + 9*(-1)^(n+1) - 11 )/12. (End)

A106256 Numbers n such that 12*n^2 + 13 is a square.

Original entry on oeis.org

1, 3, 17, 43, 237, 599, 3301, 8343, 45977, 116203, 640377, 1618499, 8919301, 22542783, 124229837, 313980463, 1730298417, 4373183699, 24099948001, 60910591323, 335668973597, 848375094823, 4675265682357, 11816340736199

Offset: 1

Pierre CAMI, Apr 28 2005

			12*1^2+13 = 5^2.
12*3^2+13 = 11^2.
12*17^2+13 = 59^2.

Index entries for linear recurrences with constant coefficients, signature (0,14,0,-1).

Cf. A106257.

Vec(x*(x+1)^3/((x^2-4*x+1)*(x^2+4*x+1)) + O(x^100)) \\ Colin Barker, Apr 16 2014

Recurrence: a(1)=1, a(2)=3, a(3)=14*a(1)+a(2), a(4)=14*a(2)+a(1) then a(n)=14*a(n-2)-a(n-4).
G.f.: x*(x+1)^3 / ((x^2-4*x+1)*(x^2+4*x+1)). - Corrected by Colin Barker, Apr 16 2014
a(2n) = 4*A007655(n)-A001570(n-1), a(2n+1) = 4*A007655(n)+A001570(n).

Edited by Ralf Stephan, Jun 01 2007

A110294 a(2n) = A028230(n), a(2n+1) = -A067900(n+1).

Original entry on oeis.org

1, -8, 15, -112, 209, -1560, 2911, -21728, 40545, -302632, 564719, -4215120, 7865521, -58709048, 109552575, -817711552, 1525870529, -11389252680, 21252634831, -158631825968, 296011017105, -2209456310872, 4122901604639, -30773756526240, 57424611447841

Offset: 0

Creighton Dement, Jul 18 2005

See A110293.

Colin Barker, Table of n, a(n) for n = 0..1000
Index entries for linear recurrences with constant coefficients, signature (0,14,0,-1).

Cf. A001353, A001570, A011943, A028230, A067900, A110293.

[(3*(-1)^n-1)*Evaluate(ChebyshevSecond(n+1), 2)/2: n in [0..40]]; // G. C. Greubel, Jan 04 2023

seriestolist(series((1-8*x+x^2)/((x^2-4*x+1)*(x^2+4*x+1)), x=0,25));

CoefficientList[Series[(1-8x+x^2)/((1-4x+x^2)(1+4x+x^2)), {x, 0, 24}], x] (* Michael De Vlieger, Nov 01 2016 *)
LinearRecurrence[{0,14,0,-1},{1,-8,15,-112},30] (* Harvey P. Dale, Dec 16 2024 *)

Vec((1-8*x+x^2)/((1-4*x+x^2)*(1+4*x+x^2)) + O(x^30)) \\ Colin Barker, Nov 01 2016

[(3*(-1)^n-1)*chebyshev_U(n,2)/2 for n in range(41)] # G. C. Greubel, Jan 04 2023

G.f.: (1-8*x+x^2) / ((1-4*x+x^2)*(1+4*x+x^2)).
a(n) = 14*a(n-2) - a(n-4) for n>3. - Colin Barker, Nov 01 2016
a(n) = (3*(-1)^n - 1)*A001353(n+1)/2. - R. J. Mathar, Sep 11 2019

A123971 Triangle T(n,k), read by rows, defined by T(n,k)=3*T(n-1,k)-T(n-1,k-1)-T(n-2,k), T(0,0)=1, T(1,0)=2, T(1,1)=-1, T(n,k)=0 if k<0 or if k>n.

Original entry on oeis.org

1, 2, -1, 5, -5, 1, 13, -19, 8, -1, 34, -65, 42, -11, 1, 89, -210, 183, -74, 14, -1, 233, -654, 717, -394, 115, -17, 1, 610, -1985, 2622, -1825, 725, -165, 20, -1, 1597, -5911, 9134, -7703, 3885, -1203, 224, -23, 1, 4181, -17345, 30691, -30418, 18633, -7329

Offset: 0

Gary W. Adamson and Roger L. Bagula, Oct 30 2006

This entry is the result of merging two sequences, this one and a later submission by Philippe Deléham, Nov 29 2013 (with edits from Ralf Stephan, Dec 12 2013). Most of the present version is the work of Philippe Deléham, the only things remaining from the original entry are the sequence data and the Mathematica program. - N. J. A. Sloane, May 31 2014
Subtriangle of the triangle given by (0, 2, 1/2, 1/2, 0, 0, 0, 0, 0, 0, 0, ...) DELTA (1, -2, 0, 0, 0, 0, 0, 0, 0, ...) where DELTA is the operator defined in A084938.
Apart from signs, equals A126124.
Row sums = 1.
Sum_{k=0..n} T(n,k)*(-x)^k = A001519(n+1), A079935(n+1), A004253(n+1), A001653(n+1), A049685(n), A070997(n), A070998(n), A072256(n+1), A078922(n+1), A077417(n), A085260(n+1), A001570(n+1) for x=0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 respectively.

			Triangle begins:
  1
  2, -1
  5, -5, 1
  13, -19, 8, -1
  34, -65, 42, -11, 1
  89, -210, 183, -74, 14, -1
  233, -654, 717, -394, 115, -17, 1
Triangle (0, 2, 1/2, 1/2, 0, 0, ...) DELTA (1, -2, 0, 0, ...) begins:
  1
  0, 1
  0, 2, -1
  0, 5, -5, 1
  0, 13, -19, 8, -1
  0, 34, -65, 42, -11, 1
  0, 89, -210, 183, -74, 14, -1
  0, 233, -654, 717, -394, 115, -17, 1

Cf. A094954, A098495, A123971, A126124, A152063, A001519, A079935, A004253, A001653, A049685, A070997, A070998, A072256, A078922, A077417, A085260, A001570, A001870, A126124.

Mathematica ( general k th center) Clear[M, T, d, a, x, k] k = 3 T[n_, m_, d_] := If[ n == m && n < d && m < d, k, If[n == m - 1 || n == m + 1, -1, If[n == m == d, k - 1, 0]]] M[d_] := Table[T[n, m, d], {n, 1, d}, {m, 1, d}] Table[M[d], {d, 1, 10}] Table[Det[M[d]], {d, 1, 10}] Table[Det[M[d] - x*IdentityMatrix[d]], {d, 1, 10}] a = Join[{M[1]}, Table[CoefficientList[ Det[M[d] - x*IdentityMatrix[d]], x], {d, 1, 10}]] Flatten[a] MatrixForm[a] Table[NSolve[Det[M[d] - x*IdentityMatrix[d]] == 0, x], {d, 1, 10}] Table[x /. NSolve[Det[M[d] - x*IdentityMatrix[d]] == 0, x][[d]], {d, 1, 10}]

T(n,k)=polcoeff(polcoeff(Ser((1-x)/(1+(y-3)*x+x^2)),n,x),n-k,y) \\ Ralf Stephan, Dec 12 2013

@CachedFunction
def A123971(n,k): # With T(0,0) = 1!
    if n< 0: return 0
    if n==0: return 1 if k == 0 else 0
    h = 2*A123971(n-1,k) if n==1 else 3*A123971(n-1,k)
    return A123971(n-1,k-1) - A123971(n-2,k) - h
for n in (0..9): [A123971(n,k) for k in (0..n)] # Peter Luschny, Nov 20 2012

T(n,k) = (-1)^n*A126124(n+1,k+1).
T(n,k) = (-1)^k*Sum_{m=k..n} binomial(m,k)*binomial(m+n,2*m). - Wadim Zudilin, Jan 11 2012
G.f.: (1-x)/(1+(y-3)*x+x^2).
T(n,0) = A001519(n+1) = A000045(2*n+1).
T(n+1,1) = -A001870(n).

Edited by N. J. A. Sloane, May 31 2014

cp's OEIS Frontend

A242394 Number of equilateral triangles (sides length = 1) that intersect the circumference of a circle of radius n centered at (0,0).

Views

Author

Keywords

Comments

Links

Crossrefs

A340295 a(n) = 4^(2*n^2) * Product_{1<=j,k<=n} (1 - sin(j*Pi/(2*n+1))^2 * cos(k*Pi/(2*n+1))^2).

Views

Author

Keywords

Comments

Links

Crossrefs

Programs

Mathematica

PARI

PARI

Formula

A006244 Hexagonal numbers (A000384) which are also centered hexagonal numbers (A003215).

Views

Author

Keywords

Comments

Examples

References

Links

Crossrefs

Programs

Maple

Mathematica

PARI

Formula

Extensions

A117808 Primes of the form ((2 + sqrt(3))^(2*k+1) + (2 - sqrt(3))^(2*k+1))/4.

Views

Author

Keywords

Comments

Links

Crossrefs

Programs

PARI

Extensions

A120744 Least k>0 such that a centered polygonal number nk(k+1)/2+1 is a perfect square; or -1 if no such number exists.

Views

Author

Keywords

Examples

Links

Crossrefs

Formula

Extensions

A088587 Difference between the two segments of edge d = A089025(n) partitioned by cevian c = A088513(n).

Views

Author

Keywords

Comments

Crossrefs

Programs

Mathematica

Formula

Extensions

A097947 Expansion of g.f. (2+7*x+2*x^2)/((x^2-1)*(1+4*x+x^2)).

Views

Author

Keywords

Comments

Links

Crossrefs

Programs

Mathematica

Formula

A106256 Numbers n such that 12*n^2 + 13 is a square.

Views

Author

Keywords

Examples

Links

A340295 a(n) = 4^(2n^2) Product_{1<=j,k<=n} (1 - sin(jPi/(2n+1))^2 * cos(kPi/(2n+1))^2).

A117808 Primes of the form ((2 + sqrt(3))^(2k+1) + (2 - sqrt(3))^(2k+1))/4.

A097947 Expansion of g.f. (2+7x+2x^2)/((x^2-1)(1+4x+x^2)).

A110294 a(2n) = A028230(n), a(2n+1) = -A067900(n+1).