%I #3 Apr 03 2013 22:49:40

%S 1,2,3,7,5,8,18,57,239,4,13,21,38,47,268,12,17,41,70,99,157,307,6,31,

%T 43,68,117,191,302,327,882,18543,9,32,73,132,278,378,829,993,2943,23,

%U 30,83,182,242,401,447,606,931,1143,1772,6118,34208,44179,85353,485298

%N Irregular triangle of numbers k such that prime(n) is the largest prime factor of k^2 + 1.

%C Note that primes of the form 4x+3 are not divisors.

%e Irregular triangle:

%e {1},

%e {},

%e {2, 3, 7},

%e {},

%e {},

%e {5, 8, 18, 57, 239},

%e {4, 13, 21, 38, 47, 268},

%e {},

%e {},

%e {12, 17, 41, 70, 99, 157, 307},

%e {},

%e {6, 31, 43, 68, 117, 191, 302, 327, 882, 18543},

%e {9, 32, 73, 132, 278, 378, 829, 993, 2943}

%t t = Table[FactorInteger[n^2 + 1][[-1,1]], {n, 10^5}]; Table[Flatten[Position[t, Prime[n]]], {n, 13}]

%Y Cf. A175607 (largest number k such that the greatest prime factor of k^2-1 is prime(n)).

%Y Cf. A223701-A223707 (related sequences).

%K nonn,tabf

%O 1,2

%A _T. D. Noe_, Apr 03 2013