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 A128815 Numbers n such that n-th and (n+2)th triangular numbers sum up to a prime. 2
 1, 2, 4, 5, 7, 11, 13, 14, 16, 19, 20, 23, 26, 32, 37, 40, 49, 53, 56, 58, 61, 65, 68, 70, 74, 76, 77, 79, 88, 89, 98, 100, 104, 109, 110, 116, 118, 130, 137, 140, 142, 146, 149, 152, 154, 160, 161, 163, 166, 167, 172, 175, 187, 188, 191, 193, 202, 205, 208, 214, 217 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Or, numbers n such that 3+3n+n^2 is prime. LINKS Robert Israel, Table of n, a(n) for n = 1..10000 FORMULA Tr(n)+Tr(n+2) is prime; A000217(n)+A000217(n+2) is prime. EXAMPLE 7 is a term because 7*8/2+9*10/2=73 is prime; 11 is a term because 11*12/2+13*14/2=157 is prime. MAPLE select(t -> isprime(3+3*t+t^2), [\$1..10000]); # Robert Israel, May 19 2014 MATHEMATICA Select[Range[300], PrimeQ[3+3#+#^2]&] Position[Partition[Accumulate[Range[250]], 3, 1], _?(PrimeQ[#[[1]]+ #[[3]]]&), 1, Heads->False]//Flatten (* Harvey P. Dale, Sep 04 2017 *) CROSSREFS Cf. A000217. Sequence in context: A099522 A255850 A108464 * A056527 A147991 A033160 Adjacent sequences:  A128812 A128813 A128814 * A128816 A128817 A128818 KEYWORD nonn AUTHOR Zak Seidov, Apr 10 2007 STATUS approved

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Last modified May 23 11:27 EDT 2022. Contains 353975 sequences. (Running on oeis4.)