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.
%I A217263 #17 Oct 09 2012 11:00:08 %S A217263 21,33,51,63,93,111,133,177,201,219,247,253,327,411,427,549,573,589, %T A217263 657,679,687,763,793,813,833,889,993,1077,1081,1119,1127,1243,1339, %U A217263 1347,1401,1411,1497,1501,1603,1623,1651,1671,1821,1839,1843,1851,1981,2009,2019 %N A217263 Composite numbers such that every concatenation of their prime factors is prime. %C A217263 The smallest term with 3 or more prime factors is 63 = 3*3*7 (see A217264). %C A217263 The smallest term with 4 or more prime factors is 21249 = 3*3*3*787 (see A217265). %C A217263 The smallest term with 5 or more prime factors is 146461 = 7*7*7*7*61. %C A217263 There is no term under 10^8 with 6 or more prime factors. %C A217263 The smallest term with 3 or more distinct prime factors is 3311 = 7*11*43 (see A180679). %C A217263 There is no term under 10^8 with 4 or more distinct prime factors. %e A217263 21 is 3*7. Both 37 and 73 are prime, so 21 is in the sequence. %e A217263 63 is 3*3*7. 337, 373 and 733 are all prime, so 63 is in the sequence. %Y A217263 Cf. A217264, A217265, A180679, A181559. Related sequences: A105184, A019549 and A106582. %K A217263 nonn,base %O A217263 1,1 %A A217263 _Dmitri Kamenetsky_, Sep 29 2012