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 A007969 Rectangular numbers. 21
 2, 5, 6, 10, 12, 13, 14, 17, 18, 20, 21, 22, 26, 28, 29, 30, 33, 34, 37, 38, 39, 41, 42, 44, 45, 46, 50, 52, 53, 54, 55, 56, 57, 58, 60, 61, 62, 65, 66, 68, 69, 70, 72, 73, 74, 76, 77, 78, 82, 84, 85, 86, 89, 90, 92, 93, 94, 95, 97, 98, 101, 102, 105, 106, 108, 109 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS A191854(n) = A007966(a(n)); A191855(n) = A007967(a(n)). - Reinhard Zumkeller, Jun 18 2011 It seems that D(n) = 4*a(n) gives precisely those even discriminants D from 4*A000037 of indefinite binary quadratic forms that have only improper solutions of the Pell equation x^2 - D*y^2 = +4. Conjecture tested for n = 1..66. Alternatively, the conjecture is that this sequence gives the r values for the Pell equation X^2 + r Y^2 = +1 whenever Y is even. See A261249 and A261250. - Wolfdieter Lang, Sep 16 2015 The proof of these two versions of the conjecture is given in the W. Lang link. - Wolfdieter Lang, Sep 19 2015 (revised Oct 03 2015) LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..200 J. H. Conway, On Happy Factorizations, J. Integer Sequences, Vol. 1, 1998, #1. Wolfdieter Lang, Proof of a Conjecture Related to the 1-Happy Numbers. FORMULA a(n) = A191854(n)*A191855(n); A007968(a(n)) = 1. - Reinhard Zumkeller, Jun 18 2011 a(n) is in the sequence if a(n) = C*B with integers B >= 1 and C >= 2, such that C*S^2 - B*R^2 = 1 has an integer solution (without loss of generality one may take S and R positive). See the Conway link. - Wolfdieter Lang, Sep 18 2015 EXAMPLE From Wolfdieter Lang, Sep 18 2015: (Start) a(1) = 5 = 5*1 and 5*1^2 - 1*2^2 = 1. a(7) = 14 = 2*7 and 2*2^2 - 7*1^2 = 1. (End) MATHEMATICA r[b_, c_] := (red = Reduce[x>0 && y>0 && b*x^2 + 1 == c*y^2, {x, y}, Integers] /. C[1] -> 1 // Simplify; If[Head[red] === Or, First[red], red]); f[128] = {}(* to speed up *); f[n_] := f[n] = If[IntegerQ[Sqrt[n]], {}, Do[c = n/b; If[(r0 = r[b, c]) =!= False, {x0, y0} = {x, y} /. ToRules[r0]; Return[{b, c, x0, y0}]], {b, Divisors[n] // Most}]]; A007969 = Reap[Table[Print[n, " ", f[n]]; If[f[n] != {} && f[n] =!= Null, Sow[n]], {n, 1, 130}]][[2, 1]] (* Jean-François Alcover, Jun 26 2012, updated Sep 18 2015 *) PROG (Haskell) a007969 n = a007969_list !! (n-1) a007969_list = filter ((== 1) . a007968) [0..] -- Reinhard Zumkeller, Oct 11 2015 CROSSREFS Every number belongs to exactly one of A000290, A007969, A007970. Cf. A191854 (B numbers), A191855 (C numbers). Cf. A007966, A007967, A007968, A261249, A261250. Subsequence of A000037, A002144 is a subsequence. A263006 (R numbers), A263007 (S numbers). Sequence in context: A064572 A032399 A276496 * A187902 A187231 A162340 Adjacent sequences: A007966 A007967 A007968 * A007970 A007971 A007972 KEYWORD nonn AUTHOR J. H. Conway STATUS approved

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Last modified July 23 12:38 EDT 2024. Contains 374549 sequences. (Running on oeis4.)