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A343827 Numbers which are the product of two S-primes (A057948) in exactly two ways. 4
441, 693, 1089, 1197, 1449, 1617, 1881, 1953, 2277, 2541, 2709, 2793, 2961, 3069, 3249, 3381, 3717, 3933, 4221, 4257, 4473, 4557, 4653, 4761, 4977, 5229, 5301, 5841, 5929, 6321, 6417, 6489, 6633, 6741, 6897, 6909, 7029, 7353, 7581, 7821, 8001, 8037, 8217, 8253 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
First differs from A057950 at a(21)=4473, whereas A057950(21)=4389, which can be represented as the product of two S-primes in exactly 3 ways.
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
FORMULA
a(n) == 1 (mod 4). - Hugo Pfoertner, May 01 2021
EXAMPLE
1449=9*161=21*69 which are all S-primes (A057948), 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)) == 2; \\ Michel Marcus, May 01 2021
CROSSREFS
Exactly one way: A343826. Exactly three ways: A343828.
Sequence in context: A252194 A057949 A057950 * A250808 A202400 A014793
KEYWORD
nonn
AUTHOR
Zachary DeStefano, Apr 30 2021
STATUS
approved

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Last modified May 24 21:23 EDT 2024. Contains 372781 sequences. (Running on oeis4.)