A223701 - OEIS

A223701

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

3, 2, 5, 7, 17, 4, 9, 11, 19, 26, 31, 49, 161, 6, 8, 13, 15, 29, 41, 55, 71, 97, 99, 127, 244, 251, 449, 4801, 8749, 10, 21, 23, 34, 43, 65, 76, 89, 109, 111, 197, 199, 241, 351, 485, 769, 881, 1079, 6049, 19601, 12, 14, 25, 27, 51, 53, 64, 79, 129, 131, 155

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

COMMENTS

Note that the first number of each row forms the sequence 3, 2, 4, 6, 10, 12,..., which is A039915. The rows, except the first, are in A181447-A181470.

LINKS

Table of n, a(n) for n=1..60.

EXAMPLE

Irregular triangle:

{3},

{2, 5, 7, 17},

{4, 9, 11, 19, 26, 31, 49, 161},

{6, 8, 13, 15, 29, 41, 55, 71, 97, 99, 127, 244, 251, 449, 4801, 8749}

MATHEMATICA

t = Table[FactorInteger[n^2 - 1][[-1, 1]], {n, 2, 10^5}]; Table[1 + Flatten[Position[t, Prime[n]]], {n, 6}]

CROSSREFS

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