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 A217755 Numbers n such that ((n^2 + n)/2)^2 + 1 is prime. 1
 1, 3, 4, 8, 11, 15, 20, 24, 28, 31, 39, 43, 48, 64, 75, 76, 79, 80, 111, 116, 120, 135, 148, 155, 160, 168, 179, 184, 200, 203, 208, 211, 216, 223, 224, 235, 243, 251, 263, 264, 288, 303, 319, 320, 324, 348, 351, 356, 364, 371, 375, 388, 416, 419, 420, 424, 428, 439, 443, 451, 456, 459 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also, numbers n such that the sum of the first n cubes precedes a prime. LINKS EXAMPLE 1 is in the sequence because ((1^2 + 1)/2)^2 + 1 = 2 and 2 is prime; 3 is in the sequence because ((3^2 + 3)/2)^2 + 1 = 37 and 37 is prime. MATHEMATICA Select[Range[400], PrimeQ[((#^2 + #)/2)^2 + 1] &] (* Alonso del Arte, Mar 26 2013 *) PROG (PARI) for(n=1, 10^3, if(isprime(((n^2 + n)/2)^2 + 1), print1(n, ", "))); /* Joerg Arndt, Mar 27 2013 */ (MAGMA) [n: n in [1..500] | IsPrime(s+1) where s is (n^2+n)^2 div 4]; // Bruno Berselli, Mar 27 2013 CROSSREFS Cf. A067186. Sequence in context: A287454 A287446 A051207 * A133363 A186423 A156056 Adjacent sequences:  A217752 A217753 A217754 * A217756 A217757 A217758 KEYWORD nonn AUTHOR Luca Brigada Villa, Mar 26 2013 STATUS approved

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Last modified October 23 16:44 EDT 2019. Contains 328373 sequences. (Running on oeis4.)