login
Multiply by 1, add 1, multiply by 2, add 2, etc., start with 2.
6

%I #24 Feb 26 2018 04:16:23

%S 2,2,3,6,8,24,27,108,112,560,565,3390,3396,23772,23779,190232,190240,

%T 1712160,1712169,17121690,17121700,188338700,188338711,2260064532,

%U 2260064544,29380839072,29380839085,411331747190,411331747204,6169976208060,6169976208075,98719619329200

%N Multiply by 1, add 1, multiply by 2, add 2, etc., start with 2.

%H Robert Israel, <a href="/A019465/b019465.txt">Table of n, a(n) for n = 0..898</a>

%F From _Robert Israel_, Dec 22 2015: (Start)

%F a(2*k) = 2*k! + Sum_{j=0..k-1} k!/j! = 2*k! + k*e*Gamma(k,1).

%F a(2*k+1) = 2*(k+1)! + Sum_{j=0..k-1} (k+1)!/j! = 2*(k+1)! + k*(k+1)*e*Gamma(k,1).

%F a(n) ~ (e+2)*(ceiling(n/2))!. (End)

%p A[0]:= 2:

%p for n from 0 to 14 do

%p A[2*n+1]:= (n+1)*A[2*n];

%p A[2*n+2]:= (n+1)+A[2*n+1];

%p od:

%p seq(A[i],i=0..30); # _Robert Israel_, Dec 22 2015

%t a = {2}; Do[If[EvenQ@ Length@ a, AppendTo[a, Floor[Length[a]/2] Last@ a],

%t AppendTo[a, Last@ a + Floor[Length[a] /2]]], {k, 27}]; Rest@ a (* _Michael De Vlieger_, Dec 22 2015 *)

%o (PARI) A019465(n,a=2)={for(i=2,n+1,if(bittest(i,0),a+=i\2,a*=i\2));a} \\ _M. F. Hasler_, Feb 25 2018

%Y Cf. A082458 (same, but start with 0), A019465 (start with 2), A019466 (start with 3).

%Y Cf. A019460 .. A019463 & A082448 (similar, but first add, then multiply).

%K nonn,easy

%O 0,1

%A _N. J. A. Sloane_

%E Edited by _M. F. Hasler_, Feb 25 2018