%I #5 Jul 29 2015 01:15:01
%S 1,2,6,30,88
%N Numbers n such that 1.2. ... .n-1.n + 1 is prime (where dot between numbers means concatenation).
%e 6 is in the sequence because 123456+1 is prime.
%t f[n_] := Block[{p = 0, k = 1}, While[k <= n, p = 10^Floor[ Log[10, k] + 1]p + k; k++ ]; PrimeQ[p + 1]]; Do[ If[ f[n], Print[n]], {n, 1000}] (* _Robert G. Wilson v_, Nov 01 2004 *)
%Y Cf. A099082, A099083.
%K base,more,nonn
%O 1,2
%A _Farideh Firoozbakht_, Oct 23 2004
|