The OEIS mourns the passing of Jim Simons and is grateful to the Simons Foundation for its support of research in many branches of science, including the OEIS.
The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

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