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 A092120 a(n) is the first term p in a sequence of primes such that p+4m^2 is prime for m = 0 to n, but composite for m = n+1; a(n) = -1 if no such prime exists. 5
 2, 19, 3, 277, 43, 53593, 7, 67, 37, 1483087, 1867783, 9671300983, 376040154163, 13491637509487, 604490757900187, 409333 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS Similar to A092474 except that a(n)+4m^2 is composite for m = n+1. a(19)=163. All other terms after a(15) are greater than 10^17 (if they exist). [From Jens Kruse Andersen, Oct 24 2008] LINKS Carlos Rivera, Puzzle 464. p+4*x^2. [From Jens Kruse Andersen, Oct 24 2008] EXAMPLE a(3) = 277 because 277, 277 + 2^2 = 281, 277 + 4^2 = 293, and 277 + 6^2 = 313 are all prime, but 277 + 8^2 = 341 = 11*31 is composite, and there is no smaller prime with this property. a(4) = 43: 43+4*1^2 = 47, which is prime. 43+4*2^2 = 59, which is prime. 43+4*3^2 = 79, which is prime. 43+4*4^2 = 107, which is prime. 43+4*5^2 = 143 = 11*13, which is composite. CROSSREFS Cf. A000040 (the prime numbers), A023200 (primes p such that p + 4 is also prime), A049492 (primes p such that p + 4 and p + 16 are also prime), A092475 (primes p such that p + 4, p + 16 and p + 36 are also prime). Sequence in context: A321340 A221229 A221602 * A059706 A128361 A096481 Adjacent sequences:  A092117 A092118 A092119 * A092121 A092122 A092123 KEYWORD nonn AUTHOR Ray G. Opao, Mar 29 2004 EXTENSIONS Correction and a(11) - a(15) from Jens Kruse Andersen, Oct 24 2008 Edited by N. J. A. Sloane, Feb 08 2019, merging this with an essentially identical sequence submitted by Jon E. Schoenfield, Feb 02 2019 STATUS approved

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Last modified May 26 02:56 EDT 2020. Contains 334613 sequences. (Running on oeis4.)