OFFSET
0,2
COMMENTS
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..80
FORMULA
G.f.: A(x) = Sum_{n>=0} x^n/(1-4^n*x).
G.f.: Sum_{n>=1} a(n)*x^n/2^(n^2) = ( Sum_{n>=0} x^n/2^(n^2) )^2. - Paul D. Hanna, Oct 14 2009
EXAMPLE
A(x) = 1/(1-x) + x/(1-4x) + x^2/(1-16x) + x^3/(1-64x) + ...
= 1 + 2*x + 6*x^2 + 34*x^3 + 386*x^4 + 8706*x^5 + ...
From Paul D. Hanna, Oct 14 2009: (Start)
Another g.f.: (1 + x/2^1 + x^2/2^4 + x^3/2^9 + x^4/2^16 + ...)^2
= 1 + 2*x/2^1 + 6*x^2/2^4 + 34*x^3/2^9 + 386*x^4/2^16 + ... (End)
MATHEMATICA
Table[Sum[4^(k*(n-k)), {k, 0, n}], {n, 0, 30}] (* G. C. Greubel, Jun 29 2021 *)
PROG
(PARI) a(n)=sum(k=0, n, (4^k)^(n-k) );
(PARI) {a(n)=2^(n^2)*polcoeff(sum(m=0, n, x^m/2^(m^2)+x*O(x^n))^2, n)} \\ Paul D. Hanna, Oct 14 2009
(Magma) [(&+[4^(k*(n-k)): k in [0..n]]): n in [0..30]]; // G. C. Greubel, Jun 29 2021
(Sage) [sum(4^(k*(n-k)) for k in (0..n)) for n in (0..30)] # G. C. Greubel, Jun 29 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Apr 15 2006
STATUS
approved