A141266 - OEIS

%I #8 Dec 07 2019 17:22:23

%S 1,12,123,1238,123815,12381548,12381548105,12381548105384,

%T 12381548105384945,123815481053849453840,12381548105384945384010395,

%U 1238154810538494538401039546080,1238154810538494538401039546080135135

%N Concatenate double factorials.

%H Florentin Smarandache <a href="http://www.gallup.unm.edu/~smarandache/SMARCON.HTM">Smarandache Concatenated Sequences</a>

%F a(n) = a(n-1)*10^floor(1+log_10(n!!)) + n!!, with a(1)=1.

%p P:=proc(i) local a,j,k,n; a:=1; print(1); for n from 2 by 1 to i do k:=n; j:=n-2; while j>0 do k:=k*j; j:=j-2; od; a:=a*10^floor(evalf(1+log10(k),1000))+k; print(a); od; end: P(20);

%Y Cf. A045508, A141265, A141267.

%K nonn,base

%O 0,2

%A _Paolo P. Lava_, Jun 20 2008