cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

A294674 Numbers that are the product of any number of consecutive odd primes.

This page as a plain text file.
%I A294674 #34 Feb 03 2018 12:18:20
%S A294674 1,3,5,7,11,13,15,17,19,23,29,31,35,37,41,43,47,53,59,61,67,71,73,77,
%T A294674 79,83,89,97,101,103,105,107,109,113,127,131,137,139,143,149,151,157,
%U A294674 163,167,173,179,181,191,193,197,199,211,221,223,227,229,233,239,241,251,257,263,269,271
%N A294674 Numbers that are the product of any number of consecutive odd primes.
%C A294674 If a(n) is an odd squarefree number with no gaps in its prime >= A065091(1) factors, b(n) is an odd squarefree number with no gaps in its prime >= A065091(2) factors, and c(n) is an odd squarefree number with no gaps in its prime >= A065091(3) factors, ..., then a(n) >= b(n) >= c(n) >= ... >= A056911(n).
%e A294674 105 is in this sequence because 105 = 3*5*7 = A065091(1)*A065091(2)*A065091(3), where A065091() are odd primes.
%t A294674 {1}~Join~Select[Range[3, 275, 2], And[SquareFreeQ@ #, MemberQ[{{}, {1}}, Union@ Differences@ PrimePi@ FactorInteger[#][[All, 1]]]] &] (* _Michael De Vlieger_, Nov 15 2017 *)
%o A294674 (PARI) isok(n) = {if ((n % 2) && issquarefree(n), f = factor(n); v = vector(#f~, k, primepi(f[k,1])); for (k=2, #v, if (v[k] - v[k-1] != 1, return (0))); return (1);); return (0);} \\ _Michel Marcus_, Nov 08 2017
%Y A294674 Intersection of A056911 and A073485.
%Y A294674 Cf. A065091, A294472.
%K A294674 nonn,easy
%O A294674 1,2
%A A294674 _Juri-Stepan Gerasimov_, Nov 06 2017
%E A294674 a(57) corrected by _Rémy Sigrist_, Nov 18 2017