A289087 Imaginary parts of the recursive sequence a(n+2) = Sum_{k=0..n} binomial(n,k)*a(k)*a(n+1-k), with a(0)=1, a(1)=2i.

0, 2, 2, 2, 2, -30, -270, -1438, -6142, -7358, 272242, 4257922, 43023682, 311000610, 695755890, -27404769758, -664760994302, -9894650252158, -102388542735118, -391004126650558, 14167871151044162, 484472962695818850, 9626237165050052850, 131917051968266058722

Offset

0,2

Comments

Here, i is the imaginary unit. The complex integer sequence c(n) = A289086(n) + i*A289087(n) is one of a family of sequences whose e.g.f.s satisfy the differential equation f''(z) = f'(z)f(z). For more details, see A289064 and A289082.

Links

Stanislav Sykora, Table of n, a(n) for n = 0..200

S. Sykora, Sequences related to the differential equation f'' = af'f, Stan's Library, Vol. VI, Jun 2017.

Formula

E.g.f.: imag(2*L0*tan(L0*z + L1)), where L0 = sqrt(i-1/4) and L1 = arccos(sqrt(1+i/4)).

Mathematica

a[0]=1; a[1]=2I; a[n_]:=a[n]=Sum[Binomial[n - 2, k] a[k] a[n - 1 - k], {k, 0, n - 2}]; Im[Table[a[n], {n, 0, 50}]] (* Indranil Ghosh, Jul 20 2017 *)

Prog

(PARI) c0=1; c1=2*I; nmax = 200;
  a=vector(nmax+1); a[1]=c0; a[2]=c1;
  for(m=0, #a-3, a[m+3]=sum(k=0, m, binomial(m, k)*a[k+1]*a[m+2-k]));
  imag(a)

Crossrefs

Cf. A289086 (real part).

Sequences for other starting pairs: A000111 (1,1), A289064 (1,-1), A289065 (2,-1), A289066 (3,1), A289067 (3,-1), A289068 (1,-2), A289069 (3,-2), A289070 (0,3), A289082 and A289083 (1,i), A289084 and A289085 (2,i), A289088 and A289089 (2,2i).

Sequence in context: A292023 A286851 A320017 * A154288 A225057 A084954

Adjacent sequences: A289084 A289085 A289086 * A289088 A289089 A289090

Keyword

sign

Author

Stanislav Sykora, Jul 19 2017

Status

approved