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 A078587 Largest prime p such that p
 3, 3, 5, 3, 5, 7, 7, 5, 11, 7, 11, 13, 13, 11, 17, 7, 17, 19, 13, 17, 19, 19, 23, 23, 19, 17, 29, 19, 23, 29, 31, 29, 31, 31, 29, 37, 37, 29, 41, 19, 41, 43, 31, 41, 43, 37, 47, 43, 43, 47, 47, 43, 53, 53, 43, 47, 59, 43, 53, 59, 61, 59, 61, 61, 53, 67, 67, 59, 71, 67, 59, 71 (list; graph; refs; listen; history; text; internal format)
 OFFSET 4,1 COMMENTS Suggested by Goldbach Conjecture. Also, values of p from A143697. This follows from the factorization n^2-k^2 = (n-k)(n+k). - T. D. Noe, Jan 22 2009 LINKS P. CAMI, Table of n, a(n) for n = 4..60000 FORMULA a(n) = 2n - A078496(n) MATHEMATICA Table[p=n+1; q=2n-p; While[q>0&&!(PrimeQ[p]&&PrimeQ[q]), p++; q-- ]; q, {n, 4, 100}] PROG PARI) a(n) = {my(p = precprime(n-1)); while(!isprime(2*n-p), p = precprime(p-1)); p; } \\ Michel Marcus, Oct 22 2016 CROSSREFS Cf. A143697, A078496. Cf. A082467 Sequence in context: A204022 A131832 A255316 * A335074 A239931 A033558 Adjacent sequences:  A078584 A078585 A078586 * A078588 A078589 A078590 KEYWORD easy,nonn AUTHOR T. D. Noe, Dec 02 2002 EXTENSIONS Edited by N. J. A. Sloane, Jan 24 2009 at the suggestion of R. J. Mathar and T. D. Noe. STATUS approved

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Last modified December 7 12:17 EST 2021. Contains 349581 sequences. (Running on oeis4.)