A100409 - OEIS

%I #7 Feb 02 2015 05:23:24

%S 48,68,78,88,98

%N Numbers n such that n is d_1*d_2*...*d_k-th composite number (n=A002808(d_1*d_2*...*d_k)) where d_1 d_2 ... d_k is the decimal expansion of n.

%C If n is a composite number then A002808(n - pi(n) - 1) = n.

%e 98 is in the sequence because 9*8=72 and 98 is 72nd composite number (98=A002808(9*8)).

%t Do[h=IntegerDigits[n];l=Length[h];If[ !PrimeQ[n]&& n-PrimePi[n]-1==Product[h[[k]], {k, l}], v=Append[v, n]; Print[n]], {n, 100000}]

%Y Cf. A002808.

%K base,fini,full,nonn

%O 1,1

%A _Farideh Firoozbakht_, Dec 06 2004