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 A031363 Positive numbers of the form x^2 + xy - y^2; or, of the form 5x^2 - y^2. 31
 1, 4, 5, 9, 11, 16, 19, 20, 25, 29, 31, 36, 41, 44, 45, 49, 55, 59, 61, 64, 71, 76, 79, 80, 81, 89, 95, 99, 100, 101, 109, 116, 121, 124, 125, 131, 139, 144, 145, 149, 151, 155, 164, 169, 171, 176, 179, 180, 181, 191, 196, 199, 205, 209, 211, 220, 225, 229, 236 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS 5x^2 - y^2 has discriminant 20, x^2 + xy - y^2 has discriminant 5. - N. J. A. Sloane, May 30 2014 Representable as x^2 + 3xy + y^2 with 0 <= x <= y. - Benoit Cloitre, Nov 16 2003 Numbers k such that x^2 - 3xy + y^2 + k = 0 has integer solutions. - Colin Barker, Feb 04 2014 Numbers k such that x^2 - 7xy + y^2 + 9k = 0 has integer solutions. - Colin Barker, Feb 10 2014 REFERENCES M. Baake, "Solution of coincidence problem ...", in R. V. Moody, ed., Math. of Long-Range Aperiodic Order, Kluwer 1997, pp. 9-44. LINKS Robert Israel and Vincenzo Librandi, Table of n, a(n) for n = 1..10000 M. Baake, Solution of the coincidence problem in dimensions d <= 4, arxiv:math/0605222 [math.MG], 2006. M. Baake and R. V. Moody, Similarity submodules and semigroups in Quasicrystals and Discrete Geometry, ed. J. Patera, Fields Institute Monographs, vol. 10 AMS, Providence, RI (1998) pp. 1-13. J. H. Conway, E. M. Rains and N. J. A. Sloane, On the existence of similar sublattices, Canad. J. Math. 51 (1999), 1300-1306 (Abstract, pdf, ps). N. J. A. Sloane, My favorite integer sequences, in Sequences and their Applications (Proceedings of SETA '98). N. J. A. Sloane et al., Binary Quadratic Forms and OEIS (Index to related sequences, programs, references) FORMULA Consists exactly of numbers in which primes == 2 or 3 mod 5 occur with even exponents. Indices of the nonzero terms in expansion of Dirichlet series Product_p (1-(Kronecker(m, p)+1)*p^(-s)+Kronecker(m, p)*p^(-2s))^(-1) for m = 5. MAPLE select(t -> nops([isolve(5*x^2-y^2=t)])>0, [\$1..1000]); # Robert Israel, Jun 12 2014 MATHEMATICA ok[n_] := Resolve[Exists[{x, y}, Element[x|y, Integers], n == 5*x^2-y^2]]; Select[Range, ok] (* or, for a large number of terms: *) max = 60755 (* max=60755 yields 10000 terms *); A031363 = {}; xm = 1; While[T = A031363; A031363 = Table[5*x^2 - y^2, {x, 1, xm}, {y, 0, Floor[ x*Sqrt]}] // Flatten // Union // Select[#, # <= max&]&; A031363 != T, xm = 2*xm]; A031363  (* Jean-François Alcover, Mar 21 2011, updated Mar 17 2018 *) PROG (PARI) select(x -> x, direuler(p=2, 101, 1/(1-(kronecker(5, p)*(X-X^2))-X)), 1) \\ Fixed by Andrey Zabolotskiy, Jul 30 2020, after hints by Colin Barker, Jun 18 2014, and Michel Marcus (PARI) is(n)=#bnfisintnorm(bnfinit(z^2-z-1), n) \\ Ralf Stephan, Oct 18 2013 (PARI) seq(M, k=3) = { \\ assume k >= 0 setintersect([1..M], setbinop((x, y)->x^2 + k*x*y + y^2, [0..1+sqrtint(M)])); }; seq(236) \\ Gheorghe Coserea, Jul 29 2018 CROSSREFS Numbers representable as x^2 + k*x*y + y^2 with 0 <= x <= y, for k=0..9: A001481(k=0), A003136(k=1), A000290(k=2), this sequence, A084916(k=4), A243172(k=5), A242663(k=6), A243174(k=7), A243188(k=8), A316621(k=9). See A035187 for number of representations. Primes in this sequence: A038872, also A141158. For a list of sequences giving numbers and/or primes represented by binary quadratic forms, see the "Binary Quadratic Forms and OEIS" link. See also the related sequence A263849 based on a theorem of Maass. Sequence in context: A117870 A162698 A166562 * A118142 A193584 A155149 Adjacent sequences:  A031360 A031361 A031362 * A031364 A031365 A031366 KEYWORD nonn,easy,nice AUTHOR EXTENSIONS More terms from Erich Friedman b-file corrected and extended by Robert Israel, Jun 12 2014 STATUS approved

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Last modified July 26 09:58 EDT 2021. Contains 346294 sequences. (Running on oeis4.)