A039721 a(1) = 1, a(m+1) = 2*Sum_{k=1..floor((m+1)/2)} a(m+1-k).

1, 2, 4, 12, 32, 96, 280, 840, 2496, 7488, 22400, 67200, 201408, 604224, 1812112, 5436336, 16307328, 48921984, 146760960, 440282880, 1320833664, 3962500992, 11887458176, 35662374528, 106986989184, 320960967552, 962882499840, 2888647499520, 8665941290112

Offset

1,2

Links

Robert Israel, Table of n, a(n) for n = 1..212

Formula

a(1)=1, a(2)=2, a(2m+1)=3*a(2m)-2*a(m), a(2m+2)=3*a(2m+1) (m is positive integer).

Example

a(6)=2*(a(5)+a(4)+a(3)) = 2*(32+12+4) = 96.

Maple

a[1]:= 1;
for m from 1 to 100 do
   a[m+1]:= 2*add(a[m+1-k], k=1..floor((m+1)/2));
od:
seq(a[i], i=1..100); # Robert Israel, May 18 2014

Mathematica

Fold[Append[#1, 2 Total[#1[[#2 - Range[Floor[#2/2] ] ]] ] ] &, {1}, Range[2, 29]] (* Michael De Vlieger, Dec 11 2017 *)

Prog

(PARI) lista(nn) = {v = vector(nn); v[1] = 1; for (n=2, nn, v[n] = 2*sum(k=1, n\2, v[n-k]); ); v; } \\ Michel Marcus, May 18 2014

Crossrefs

Cf. A039722 (similar definition).

Sequence in context: A231295 A087211 A161177 * A148193 A341344 A002528

Adjacent sequences: A039718 A039719 A039720 * A039722 A039723 A039724

Keyword

easy,nonn

Author

Leroy Quet, Dec 11 1999

Extensions

More terms from James A. Sellers, May 04 2000

Two more terms from Michel Marcus, May 18 2014

Status

approved