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 A089270 Positive numbers represented by the integer binary quadratic form x^2 + x*y - y^2 with x and y relatively prime. 19
 1, 5, 11, 19, 29, 31, 41, 55, 59, 61, 71, 79, 89, 95, 101, 109, 121, 131, 139, 145, 149, 151, 155, 179, 181, 191, 199, 205, 209, 211, 229, 239, 241, 251, 269, 271, 281, 295, 305, 311, 319, 331, 341, 349, 355, 359, 361, 379, 389, 395, 401, 409, 419, 421, 431 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The negative numbers represented by x^2 + x*y - y^2 with relative prime x and y are -a(n). The discriminant of this binary form is D=5>0, hence this is an indefinite form. It appears that these are also the n for which the equation x^2 = x+1 (mod n) has solutions. The number of solutions is 0 or a power of 2. It appears that n=5 is the only n for which x^2 = x+1 (mod n) has just one solution. The first n producing 4 solutions is 209. The first n producing 8 solutions is 6061. - T. D. Noe, Nov 04 2009 [For a proof see the W. Lang link, Proposition. - Wolfdieter Lang, Jul 04 2019] (Conjecture) The terms are the products of primes congruent to {0,1,4} mod 5 (using at most a single 5, but repeating other primes is allowed), which is A038872. - T. D. Noe, Nov 14 2010 [Comment in brackets from Shreevatsa R, Mar 27 2019. For a proof see the W. Lang link, Lemma 1, iii) and the Proposition. - Wolfdieter Lang, Jul 04 2019] Brousseau's paper lists these numbers (less than 1000) as discriminants of Fibonacci sequences. For each number, he also lists the (a,b) pairs that are the first two terms of a unique Fibonacci sequence. From Matthew Staller, Oct 01 2015: (Start) The number of fundamental solutions to n=|y^2-x^2-xy| with relatively prime x and y is 0 or 2^k, where k is the number of distinct prime factors of n that are congruent to {1,4} mod 5 (conjecture). For example, n=187891=11*19*29*31 has 16=2^4 solutions; the prime n=9999999929 has 2=2^1 solutions; n=84182245951=31^3*41^4 has 4=2^2 solutions.[For a proof of the conjecture see the W. Lang link, Lemma 1, iii) and the Proposition. - Wolfdieter Lang, Jul 04 2019] Recurring sequences (as Fibonacci sequences) may be ordered by determinant (|y^2-x^2-xy| for consecutive (x,y) terms), and further by individual terms to clarify where necessary. For example, the four distinct sequences that have a determinant of 209 are (13,8),(13,5),(14,13),(14,1) which shows how they were found but which would be more commonly understood as (8,21),(5,18),(13,27),(1,15). For a determinant of 1 there is exactly one sequence (Fibonacci, A000045), for a determinant of 5 there is just one (Lucas, A000032). For 11 there are two (1,4) and (2,5), the latter of which is known as the Evangelist sequence (A001060). (End) The linear map (x,y) -> (5x+8y, 8x+13y) maps coprime integer solutions of x^2+xy-y^2 = n to coprime integer solutions, so if there is such a solution with nonnegative x,y there must be one with y < 8 sqrt(n). - Robert Israel, Oct 01 2015 Odd numbers n such that 5 is a square mod n. - Shreevatsa R, Mar 27 2019 [For a proof see the W. Lang link, Lemma 1, iii) and Proposition. - Wolfdieter Lang, Jul 04 2019] Let m = a^2 + a*b - b^2 and n = c^2 + c*d - d^2, where gcd(a, b) = gcd(c, d) = 1. If a*d - b*c = 1, then A165900(a*c + a*d - b*d) = m*n. - Isaac Saffold, Feb 23 2020 LINKS T. D. Noe, Table of n, a(n) for n=1..1000, matches Jagy's program output (R. J. Mathar, Sep 10 2016) Alfred Brousseau, On the ordering of Fibonacci sequences, Fib. Quart. 1.4 (1963), 43-46; Errata, Fib. Quart. 2.1 (1964), 38. V. E. Hoggatt, Jr., 7-page typed letter to N. J. A. Sloane with suggestions for new sequences, circa 1977. N. J. A. Sloane et al., Binary Quadratic Forms and OEIS (Index to related sequences, programs, references) FORMULA a(n) = x^2 + x*y - y^2 with relatively prime integers x and y (proper solutions of the Diophantine equation). EXAMPLE n=2: a(2)=5 with, for example, (x,y)= (2,1): 4+2-1=5 (there are infinitely many proper (x,y) solutions). n=8: a(8)=55 with, for example, (x,y)=(7,6) or (7,1). In this case there exist two fundamental proper solutions. MAPLE F:= proc(n) local x, y;       for y from 1 to floor(8*sqrt(n)) do          x := (-y+sqrt(5*y^2+4*n))/2;          if x::integer and igcd(x, y) = 1 then return true fi;       od:       false end proc: select(F, [\$1..1000]); # Robert Israel, Oct 01 2015 MATHEMATICA max = 500; xmax = Sqrt[max] // Ceiling; Clear[repQ]; repQ[_] = False; Do[ If[GCD[x, y] == 1, repQ[x^2 + x*y - y^2] = True], {y, 0, xmax-1}, {x, y, xmax}]; Select[Range[max], repQ] (* Jean-François Alcover, Jul 17 2013 *) [Caution: brute force search! - N. J. A. Sloane, Jun 11 2014] Reap[For[n = 1, n < 1000, n++, r = Reduce[x^2 + x y - y^2 == n, {x, y}, Integers]; If[r =!= False, If[AnyTrue[{x, y} /. {ToRules[r /. C -> 0]}, CoprimeQ @@ # &], Print[n]; Sow[n]]]]][[2, 1]] (* Jean-François Alcover, Oct 31 2016 *) CROSSREFS Odd numbers in A057762. Sequence in context: A048217 A336015 A132087 * A275068 A038872 A141158 Adjacent sequences:  A089267 A089268 A089269 * A089271 A089272 A089273 KEYWORD nonn AUTHOR Wolfdieter Lang, Nov 07 2003 EXTENSIONS The Mma program from Alcover appears to use brute force search, which is notoriously unreliable for indefinite forms. - N. J. A. Sloane, Jun 11 2014 Second Mma program without brute force by Jean-François Alcover, Oct 31 2016 Minor edits by Matthew Staller, Jun 05 2019 STATUS approved

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Last modified April 21 15:55 EDT 2021. Contains 343156 sequences. (Running on oeis4.)