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A133321
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Inserting any (identical) digit between adjacent digits of an odd semiprime k never yields a prime.
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0
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15, 25, 35, 55, 65, 85, 95, 115, 121, 143, 145, 155, 185, 187, 205, 215, 235, 253, 265, 295, 299, 305, 335, 341, 355, 365, 393, 395, 411, 415, 437, 445, 451, 473, 485, 505, 515, 535, 545, 565, 583, 635, 655, 671, 679, 685, 695, 717, 745, 755, 781, 785, 815
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| Odd semiprime analogue of A050805. Trivially true for any digit if we substitute "even semiprime" for "odd semiprime." Trivially true for any semiprime which is a multiple of 5 (A001750). The nonmultiples of 5 in this sequence begin 121, 143, 187, 253, 299, 341.
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EXAMPLE
| 121 is in the sequence because 10201, 11211, 12221, 13231, 14241, 15251, 16261, 17271, 18281, 19291 are all composite.
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CROSSREFS
| Cf. A001358, A001750, A050674-A050719, A050805, A050806.
Sequence in context: A171133 A152246 A069823 * A031888 A046291 A020302
Adjacent sequences: A133318 A133319 A133320 * A133322 A133323 A133324
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KEYWORD
| base,easy,nonn
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AUTHOR
| Jonathan Vos Post (jvospost3(AT)gmail.com), Oct 18 2007
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EXTENSIONS
| More terms from R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Oct 22 2007
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