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A341660
Primes p such that p^2 - 1 has fewer than 32 divisors.
3
2, 3, 5, 7, 11, 13, 17, 19, 23, 31, 37, 47, 73
OFFSET
1,1
COMMENTS
For all primes p > 73, p^2 - 1 has at least A309906(2)=32 divisors.
EXAMPLE
p = factorization
n a(n) p^2 - 1 of p^2 - 1 tau(p^2 - 1)
-- ---- ------- -------------- ------------
1 2 3 3 2
2 3 8 2^3 4
3 5 24 2^3 * 3 8
4 7 48 2^4 * 3 10
5 11 120 2^3 * 3 * 5 16
6 13 168 2^3 * 3 * 7 16
7 17 288 2^5 * 3^2 18
8 19 360 2^3 * 3^2 * 5 24
9 23 528 2^4 * 3 * 11 20
10 31 960 2^6 * 3 * 5 28
11 37 1368 2^3 * 3^2 * 19 24
12 47 2208 2^5 * 3 * 23 24
13 73 5328 2^4 * 3^2 * 37 30
MATHEMATICA
Select[Range[100], PrimeQ[#] && DivisorSigma[0, #^2 - 1] < 32 &] (* Amiram Eldar, Feb 26 2021 *)
CROSSREFS
KEYWORD
nonn,fini,full
AUTHOR
Jon E. Schoenfield, Feb 26 2021
STATUS
approved