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%I #11 Dec 07 2019 17:22:29
%S 1,21,321,8321,158321,48158321,10548158321,38410548158321,
%T 94538410548158321,384094538410548158321,10395384094538410548158321,
%U 4608010395384094538410548158321,1351354608010395384094538410548158321
%N Concatenate first n double factorials in reverse order.
%F a(n) = n!!*10^floor(1+log_10(a(n-1))) + a(n-1), 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:=k*10^floor(evalf(1+log10(a),1000))+a; print(a); od; end: P(20);
%t FromDigits/@(Flatten[IntegerDigits/@Reverse[#]]&/@Table[ Take[Range[20]!!,n],{n,20}]) (* _Harvey P. Dale_, Sep 30 2011 *)
%Y Cf. A045508, A141265, A141266.
%K nonn,base
%O 0,2
%A _Paolo P. Lava_, Jun 20 2008