A174418 - OEIS

%I #8 Feb 26 2020 20:18:42

%S 2,3,5,15,20,300,320,96000,96320,9246720000,9246816320,

%T 85502721402470400000,85502721411717216320,

%U 7310715368019097765567999459196928000000

%N Alternately sum and multiply with a(1) = 2 and a(2) = 3.

%C a(n) | a(n+2).

%C a(n) is approximately c^(2^floor(n/2)) with c = 2.048505746253657024938.... [_Charles R Greathouse IV_, Nov 07 2011]

%F a(2n+1) = a(2n) + a(2n-1), a(2n) = a(2n-1) * a(2n-2).

%t nxt[{n_,a_,b_}]:={n+1,b,If[OddQ[n],a+b,a*b]}; NestList[nxt,{1,2,3},15][[All,2]] (* _Harvey P. Dale_, Feb 26 2020 *)

%o (PARI) a=[2,3]; for(i=1, 9, a=concat(a,[a[#a]+a[#a-1],a[#a]*(a[#a]+a[#a-1])])); a

%K easy,nonn

%O 1,1

%A _Giovanni Teofilatto_, Mar 19 2010