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 A217263 Composite numbers such that every concatenation of their prime factors is prime. 4
 21, 33, 51, 63, 93, 111, 133, 177, 201, 219, 247, 253, 327, 411, 427, 549, 573, 589, 657, 679, 687, 763, 793, 813, 833, 889, 993, 1077, 1081, 1119, 1127, 1243, 1339, 1347, 1401, 1411, 1497, 1501, 1603, 1623, 1651, 1671, 1821, 1839, 1843, 1851, 1981, 2009, 2019 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The smallest term with 3 or more prime factors is 63 = 3*3*7 (see A217264). The smallest term with 4 or more prime factors is 21249 = 3*3*3*787 (see A217265). The smallest term with 5 or more prime factors is 146461 = 7*7*7*7*61. There is no term under 10^8 with 6 or more prime factors. The smallest term with 3 or more distinct prime factors is 3311 = 7*11*43 (see A180679). There is no term under 10^8 with 4 or more distinct prime factors. LINKS EXAMPLE 21 is 3*7. Both 37 and 73 are prime, so 21 is in the sequence. 63 is 3*3*7. 337, 373 and 733 are all prime, so 63 is in the sequence. CROSSREFS Cf. A217264, A217265, A180679, A181559. Related sequences: A105184, A019549 and A106582. Sequence in context: A279229 A141249 A026068 * A176945 A070006 A189986 Adjacent sequences:  A217260 A217261 A217262 * A217264 A217265 A217266 KEYWORD nonn,base AUTHOR Dmitri Kamenetsky, Sep 29 2012 STATUS approved

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Last modified October 22 08:00 EDT 2019. Contains 328315 sequences. (Running on oeis4.)