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Search: id:A057128
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| A057128 |
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Numbers n such that -3 is a square mod n. |
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+0
6
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| 1, 2, 3, 4, 6, 7, 12, 13, 14, 19, 21, 26, 28, 31, 37, 38, 39, 42, 43, 49, 52, 57, 61, 62, 67, 73, 74, 76, 78, 79, 84, 86, 91, 93, 97, 98, 103, 109, 111, 114, 122, 124, 127, 129, 133, 134, 139, 146, 147, 148, 151, 156, 157, 158, 163, 169, 172, 181, 182, 183, 186, 193
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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The fact that there are no numbers in this sequence of the form 6k+5 leads to the result that all prime factors of central polygonal numbers (A002061 of the form n^2-n+1) are either 3 or of the form 6k+1. This in turn leads to there being an infinite number of primes of the form 6k+1, since if P=product[all known primes of form 6k+1] then all the prime factors of 9P^2-3P+1 must be unknown primes of form 6k+1.
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EXAMPLE
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a(7)=13 since -3 mod 13=10 mod 13=6^2 mod 13.
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CROSSREFS
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Includes the primes in A045331 and these (primes congruent to {1, 2, 3} mod 6) are the prime factors of the terms in this sequence. Cf. A008784, A057125, A057126, A057127, A057129.
Sequence in context: A091336 A002235 A030705 this_sequence A018534 A018276 A057732
Adjacent sequences: A057125 A057126 A057127 this_sequence A057129 A057130 A057131
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KEYWORD
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nonn
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AUTHOR
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Henry Bottomley (se16(AT)btinternet.com), Aug 10 2000
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