A343826 - OEIS

A343826

Numbers which are the product of two S-primes (A057948) in exactly one way.

25, 45, 65, 81, 85, 105, 117, 145, 153, 165, 169, 185, 189, 205, 221, 245, 261, 265, 273, 285, 289, 297, 305, 333, 345, 357, 365, 369, 377, 385, 429, 445, 465, 477, 481, 485, 493, 505, 513, 533, 545, 549, 561, 565, 605, 609, 621, 629, 637, 645, 657, 665, 685

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OFFSET

1,1

COMMENTS

There exist numbers which are the product of two S-primes in exactly 1, 2, and 3 ways; however, it is unknown if any numbers exist which are the product of two S-primes in exactly 4 ways.

LINKS

Zachary DeStefano, Table of n, a(n) for n = 1..3786

FORMULA

a(n) == 1 (mod 4). - Hugo Pfoertner, May 01 2021

EXAMPLE

153 = 9*17 which are both S-primes, and admits no other S-prime factorizations.

PROG

(PARI) \\ uses is(n) from A057948

isok(n) = sumdiv(n, d, (d<=n/d) && is(d) && is(n/d)) == 1; \\ Michel Marcus, May 01 2021

CROSSREFS

Cf. A054520, A057948, A057949, A057950.

Exactly two ways: A343827. Exactly three ways: A343828.