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A358881
a(n) is the smallest prime p such that p^2 - 1 has 2*n divisors, or -1 if no such prime exists.
0
2, 3, -1, 5, 7, -1, -1, 11, 17, 23, -1, 19, -1, 31, 73, 29, -1, 383, -1, 41, 97, -1, -1, 79, -1, -1, 127, 223, -1, 71, -1, 109, -1, -1, 2593, 197, -1, -1, -1, 281, -1, 1439, -1, 34303, 199, -1, -1, 181, -1, 647, -1, 6143, -1, 7057, -1, 929, -1, -1, -1, 521, -1
OFFSET
1,1
EXAMPLE
For p = 11, p^2 - 1 = 121 - 1 = 120 = 2^3 * 3 * 5 has 16 divisors. 11 is the smallest prime p such that p^2 - 1 has 16 = 2*8 divisors, so a(8) = 11.
There does not exist any prime p such that p^2 - 1 has 6 = 2*3 divisors, so a(3) = -1.
CROSSREFS
KEYWORD
sign
AUTHOR
Jon E. Schoenfield, Dec 04 2022
STATUS
approved