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 A257474 Number of unordered ways to write n = a + b, where a and b are distinct elements of the set {floor(x/3): 3*x-1 and 3*x+1 are twin prime}, one of a and b is even, and one of a and b has the form p-1 or p-2 with p prime. 2
 1, 1, 2, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 2, 3, 2, 3, 3, 3, 3, 5, 3, 3, 3, 5, 3, 3, 3, 5, 3, 4, 3, 3, 3, 6, 5, 1, 2, 5, 4, 2, 1, 2, 3, 4, 3, 4, 5, 3, 3, 3, 3, 3, 2, 2, 2, 4, 3, 3, 2, 3, 3, 1, 3, 4, 4, 5, 4, 4, 3, 4, 3, 3, 1, 5, 3, 5, 3, 2, 1, 3, 3, 3, 1, 2, 2, 4, 2, 4, 4, 5, 3, 4, 4, 5, 3, 3, 2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Conjecture: a(n) > 0 for all n > 0, and a(n) = 1 only for n = 1, 2, 4, 39, 44, 65, 76, 82, 86, 108, 110, 123, 154, 175, 178, 196, 205, 221, 284, 308, 621, 735, 4655. This is much stronger than the Twin Prime Conjecture. Note that a(n) <= A257317(n) <= A256707(n). We have verified that a(n) > 0 for all n = 1..10^7. LINKS Zhi-Wei Sun, Table of n, a(n) for n = 1..20000 Zhi-Wei Sun, Natural numbers represented by floor(x^2/a)+floor(y^2/b)+floor(z^2/c), arXiv:1504.01608 [math.NT], 2015. EXAMPLE a(205) = 1 since 205 = 25 + 180 = floor(76/3) + floor(540/3) with 180 even and 180 + 1 prime, and {3*76-1,3*76+1} = {227,229} and {3*540-1,3*540+1} = {1619,1621} twin prime pairs. a(308) = 1 since 308 = 128 + 180 = floor(384/3) + floor(540/3) with 180 + 1 prime, and {3*128-1,3*128+1} = {1151,1153} and {3*540-1,3*540+1} = {1619,1621} twin prime pairs. a(621) = 1 since 621 = 310 + 311 = floor(930/3) + floor(934/3) with 310 even and 310 + 1 prime, {3*930-1,3*930+1} ={2789,2791} and {3*934-1,3*934+1} = {2801,2803} twin prime pairs. a(735) = 1 since 735 = 311 + 424 = floor(934/3) + floor(1274/3) with 424 even, 311 + 2 = 313 prime, and {3*934-1,3*934+1} = {2801,2803} and {3*1274-1,3*1274+1} = {3821,3823} twin prime pairs. a(4655) = 1 since 4655 = 15 + 4640 = floor(46/3) + floor(13920/3) with 4640 even, 15 + 2 prime, and {3*46-1,3*46+1} = {137,139} and {3*13920-1,3*13920+1} = {41759,41761} twin prime pairs. MATHEMATICA TQ[n_]:=PrimeQ[3n-1]&&PrimeQ[3n+1] PQ[n_]:=TQ[3*n]||TQ[3*n+1]||TQ[3n+2] RQ[n_]:=PrimeQ[n+1]||PrimeQ[n+2] Do[r=0; Do[If[Mod[x(n-x), 2]==0&&(RQ[x]||RQ[n-x])&&PQ[x]&&PQ[n-x], r=r+1], {x, 0, (n-1)/2}]; Print[n, " ", r]; Continue, {n, 1, 100}] CROSSREFS Cf. A000040, A014574, A256707, A257121, A257317. Sequence in context: A237684 A130634 A274828 * A257317 A163376 A261913 Adjacent sequences:  A257471 A257472 A257473 * A257475 A257476 A257477 KEYWORD nonn AUTHOR Zhi-Wei Sun, Apr 25 2015 STATUS approved

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Last modified November 26 11:07 EST 2020. Contains 338639 sequences. (Running on oeis4.)