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Search: id:A109904
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| A109904 |
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a(1)= 5. a(n+1) = greatest prime of the form k*{a(n)-k} +1. The least prime occurs for k = 1 and a(n+1) = a(n) in that case if no other value of k gives a prime then the sequence terminates. |
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+0
6
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| 5, 7, 13, 43, 463, 53593, 718052371, 128899801874680399, 4153789730832965126116749598699801, 4313492281993349218329412357362100514520987205269104143837429352069
(list; graph; listen)
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OFFSET
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0,1
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COMMENT
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For the first five terms k = INT[a(n)/2]. k can take values from 1 to INT[a(n)/2]. It is conjectured that at least one value of k, 2= < k < INT[a(n)/2] gives a prime and the sequence is infinite.
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EXAMPLE
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a(2)= 2*3 +1=7, a(3) = 3*4 +1 =13.
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PROGRAM
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(PARI) { b(n)=forstep(k=n\2, 1, -1, if(isprime(k*(n-k)+1), return(k*(n-k)+1))); return(0) } s=5; while(1, print1(s, " "); s=b(s)) (Alekseyev)
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CROSSREFS
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Cf. A109905, A026728.
Sequence in context: A018464 A063446 A064600 this_sequence A077781 A102872 A102873
Adjacent sequences: A109901 A109902 A109903 this_sequence A109905 A109906 A109907
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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 (maxal(AT)cs.ucsd.edu), Oct 04 2005
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