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Search: id:A109905
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| A109905 |
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a(n) = greatest prime of the form k*(n-k) +1. 0 if no such prime exists. |
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+0
6
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| 0, 2, 3, 5, 7, 0, 13, 17, 19, 17, 31, 37, 43, 41, 37, 61, 73, 73, 89, 101, 109, 113, 131, 109, 157, 89, 181, 197, 211, 0, 241, 257, 271, 281, 307, 181, 337, 353, 379, 401, 421, 433, 463, 449, 487, 521, 547, 577, 601, 617, 631, 677, 701, 0, 757, 769, 811, 761, 859, 757
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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k can take values from 1 to floor[n/2].
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EXAMPLE
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a(15) = 37 as 1*14 +1 = 16, 2*13 +1 = 27 are composite but 3*12 +1= 37 is a prime.
a(6) = 0 as 1*5 +1=6, 2*4 +1=9, 3*3 +1 = 10 are all composite.
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PROGRAM
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(PARI) { a(n) = forstep(k=n\2, 1, -1, if(isprime(k*(n-k)+1), return(k*(n-k)+1))); return(0) } (Alekseyev)
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CROSSREFS
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Cf. A109904, A026728.
Sequence in context: A104250 A020919 A126053 this_sequence A113493 A060420 A077648
Adjacent sequences: A109902 A109903 A109904 this_sequence A109906 A109907 A109908
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KEYWORD
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nonn
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AUTHOR
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Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Jul 15 2005
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EXTENSIONS
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More terms from Max Alekseyev (maxale(AT)gmail.com), Oct 04 2005
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