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Search: id:A141274
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| A141274 |
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Composite numbers n such that n+1 is prime and the index in the composites is also prime. |
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+0
3
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| 12, 28, 36, 60, 100, 130, 138, 150, 172, 180, 198, 222, 270, 292, 348, 396, 420, 432, 456, 520, 540, 568, 612, 618, 640, 682, 732, 768, 786, 820, 838, 880, 928, 996, 1032, 1060, 1068, 1090, 1096, 1212, 1222, 1276, 1300, 1306, 1320, 1372, 1428, 1486, 1548
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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Suggested by Carlos Rivera's Puzzle 449, The Prime Puzzles & Problems Connection
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FORMULA
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Compute the composite numbers beginning with 4. At 4 the composite index is zero, at 6 it is 1, at 8 it is 2, ... and at 12 it is 5, etc.
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EXAMPLE
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a(1)=12 because 12+1=13, prime and the index of 12 is 5, so both the composite 12+1 and the index are prime.
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CROSSREFS
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Cf. A141275.
Sequence in context: A079705 A068721 A087252 this_sequence A134618 A108405 A044073
Adjacent sequences: A141271 A141272 A141273 this_sequence A141275 A141276 A141277
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KEYWORD
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easy,nonn
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AUTHOR
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Enoch Haga (Enokh(AT)comcast.net), Jun 21 2008
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EXTENSIONS
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Edited by N. J. A. Sloane (njas(AT)research.att.com), Jun 21 2008
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