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A125255
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Smallest prime divisor of 4n-1.
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2
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3, 7, 11, 3, 19, 23, 3, 31, 5, 3, 43, 47, 3, 5, 59, 3, 67, 71, 3, 79, 83, 3, 7, 5, 3, 103, 107, 3, 5, 7, 3, 127, 131, 3, 139, 11, 3, 151, 5, 3, 163, 167, 3, 5, 179, 3, 11, 191, 3, 199, 7, 3, 211, 5, 3, 223, 227, 3, 5, 239, 3, 13, 251, 3, 7, 263, 3, 271, 5, 3, 283, 7, 3, 5, 13, 3, 307
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| All divisors of 4n-1 are odd.
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LINKS
| N. Hobson, Table of n, a(n) for n = 1..1000
N. Hobson, Home page (listed in lieu of email address)
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FORMULA
| a(3k+1) = 3, k = 0,1,2,... a(5k+4) = 5 if k is not a multiple of 3; k = 1,2,4,5,7,8,... - Alexander Adamchuk (alex(AT)kolmogorov.com), Nov 28 2006
a(n) = A020639(4n-1). - R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Jan 23 2007
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EXAMPLE
| The prime divisors of 4*9-1=35 are 5 and 7, so a(9) = 5.
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PROG
| (PARI) vector(77, n, factor(4*n-1)[1, 1])
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CROSSREFS
| Cf. A004767, A107745, A125254.
Sequence in context: A042229 A042373 A042775 * A125254 A093931 A153788
Adjacent sequences: A125252 A125253 A125254 * A125256 A125257 A125258
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KEYWORD
| easy,nonn
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AUTHOR
| Nick Hobson Nov 26 2006
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