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A217755
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Numbers n such that ((n^2 + n)/2)^2 + 1 is prime.
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1
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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
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OFFSET
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1,2
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COMMENTS
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Also, numbers n such that the sum of the first n cubes precedes a prime.
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LINKS
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EXAMPLE
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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.
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MATHEMATICA
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Select[Range[400], PrimeQ[((#^2 + #)/2)^2 + 1] &] (* Alonso del Arte, Mar 26 2013 *)
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PROG
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(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
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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