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 A109904 a(1) = 5. a(n+1) is the 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. 6
 5, 7, 13, 43, 463, 53593, 718052371, 128899801874680399, 4153789730832965126116749598699801, 4313492281993349218329412357362100514520987205269104143837429352069 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS For the first five terms k = floor(a(n)/2). k can take values from 1 to floor(a(n)/2). It is conjectured that at least one value of k, 2 <= k < floor(a(n)/2) gives a prime and the sequence is infinite. LINKS Jinyuan Wang, Table of n, a(n) for n = 1..13 EXAMPLE a(2) = 2*3 + 1 = 7, a(3) = 3*4 + 1 = 13. PROG (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)) \\ Max Alekseyev, Oct 04 2005 CROSSREFS Cf. A026728, A109905. Sequence in context: A339775 A064600 A174874 * A077781 A102872 A102873 Adjacent sequences: A109901 A109902 A109903 * A109905 A109906 A109907 KEYWORD nonn AUTHOR Amarnath Murthy, Jul 15 2005 EXTENSIONS More terms from Max Alekseyev, Oct 04 2005 STATUS approved

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Last modified February 27 14:46 EST 2024. Contains 370376 sequences. (Running on oeis4.)