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 A228431 Number of ordered ways to write n = x + y (x, y > 0) with p(3, x) + p(6, y) prime, where p(3, k) denotes the triangular number k*(k+1)/2 and p(6, k) denotes the hexagonal number k*(2*k-1) = p(3, 2*k-1). 4
 0, 1, 1, 1, 2, 1, 1, 1, 2, 2, 2, 4, 4, 1, 4, 3, 1, 2, 3, 2, 5, 5, 3, 2, 3, 2, 4, 3, 3, 8, 4, 1, 3, 2, 2, 11, 5, 1, 5, 5, 4, 4, 5, 4, 7, 4, 3, 7, 6, 3, 9, 4, 2, 5, 4, 3, 12, 7, 2, 4, 10, 1, 7, 8, 4, 10, 7, 3, 10, 9, 5, 8, 5, 4, 10, 9, 5, 10, 9, 3, 12, 13, 4, 4, 9, 4, 11, 10, 5, 11, 16, 5, 10, 8, 5, 16, 8, 3, 11, 15 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,5 COMMENTS Conjecture: a(n) > 0 for all n > 1. This implies that there are infinitely many primes each of which can be written as a sum of a triangular number and a hexagonal number. See also A228425, A228428, A228429 and A228430 for more similar conjectures. LINKS Zhi-Wei Sun, Table of n, a(n) for n = 1..10000 EXAMPLE a(14) = 1 since 14 = 10 + 4 with p(3, 10) + p(6, 4) = 83 prime. a(38) = 1 since 38 = 31 + 7 with p(3, 31) + p (6, 7) = 587 prime. MATHEMATICA p[m_, x_]:=(m-2)x(x-1)/2+x a[n_]:=Sum[If[PrimeQ[p[3, x]+p[6, n-x]], 1, 0], {x, 1, n-1}] Table[a[n], {n, 1, 100}] CROSSREFS Cf. A000040, A000217, A000384, A228425, A228428, A228429, A228430. Sequence in context: A289493 A324341 A271325 * A165620 A128494 A257696 Adjacent sequences:  A228428 A228429 A228430 * A228432 A228433 A228434 KEYWORD nonn AUTHOR Zhi-Wei Sun, Nov 10 2013 STATUS approved

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Last modified October 17 11:26 EDT 2019. Contains 328108 sequences. (Running on oeis4.)