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A343827 Numbers which are the product of two S-primes (A057948) in exactly two ways. 4

%I #19 May 26 2021 02:53:37

%S 441,693,1089,1197,1449,1617,1881,1953,2277,2541,2709,2793,2961,3069,

%T 3249,3381,3717,3933,4221,4257,4473,4557,4653,4761,4977,5229,5301,

%U 5841,5929,6321,6417,6489,6633,6741,6897,6909,7029,7353,7581,7821,8001,8037,8217,8253

%N Numbers which are the product of two S-primes (A057948) in exactly two ways.

%C 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.

%C 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.

%H Zachary DeStefano, <a href="/A343827/b343827.txt">Table of n, a(n) for n = 1..2484</a>

%F a(n) == 1 (mod 4). - _Hugo Pfoertner_, May 01 2021

%e 1449=9*161=21*69 which are all S-primes (A057948), and admits no other S-prime factorizations.

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

%o isok(n) = sumdiv(n, d, (d<=n/d) && is(d) && is(n/d)) == 2; \\ _Michel Marcus_, May 01 2021

%Y Cf. A054520, A057948, A057949, A057950.

%Y Exactly one way: A343826. Exactly three ways: A343828.

%K nonn

%O 1,1

%A _Zachary DeStefano_, Apr 30 2021

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Last modified June 15 08:58 EDT 2024. Contains 373402 sequences. (Running on oeis4.)