A219761 - OEIS

%I #23 Dec 05 2012 03:24:59

%S 1,2,6,156,4260,117306,160650,13937550,32742516,3306719796,7746764190

%N a(1) = 1; for n>1, a(n) = smallest integer > a(n-1) such that a(n)*a(n-i)+1 is prime for all 0 <= i <= n-1.

%D _Rainer Rosenthal_, Posting to Sequence Fans Mailing List, Nov 30 2012.

%e After a(1)=1, a(2)=2, a(3)=6, we want the smallest m>6 such that 1+m, 1+2m, 1+6m and 1+m^2 are all prime: this is m = 156 = a(4).

%t f[a_List] := Block[{m = a, k = a[[-1]] + 6}, While[ Union@ PrimeQ[k*Join[m, {k}] + 1] != {True}, k += 6]; k]; s = {1, 2, 6}; Do[ Print[{n, a = f[s]}]; s = Append[s, a], {n, 4, 9}] (* _Robert G. Wilson v_, Dec 03 2012 *)

%Y Cf. A034881, A081942, A093483.

%K nonn,more

%O 1,2

%A _N. J. A. Sloane_, Dec 01 2012

%E a(8) and a(9) from _Robert G. Wilson v_, Dec 03 2012

%E a(10) and a(11) from _Robert G. Wilson v_, Dec 04 2012