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

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.

Sequence in context: A015911 A188005 A054520 * A339958 A192261 A038811

Adjacent sequences: A343823 A343824 A343825 * A343827 A343828 A343829

Keyword

nonn

Author

Zachary DeStefano, Apr 30 2021

Status

approved