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 A030640 Discriminant of lattice A_n of determinant n+1. 1
 1, 1, -3, -2, 5, 3, -7, -4, 9, 5, -11, -6, 13, 7, -15, -8, 17, 9, -19, -10, 21, 11, -23, -12, 25, 13, -27, -14, 29, 15, -31, -16, 33, 17, -35, -18, 37, 19, -39, -20, 41, 21, -43, -22, 45, 23, -47, -24, 49, 25, -51, -26, 53, 27, -55, -28, 57, 29, -59 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 REFERENCES J. H. Conway, The Sensual Quadratic Form, Mathematical Association of America, 1997, p. 4. G. L. Watson, Integral Quadratic Forms, Cambridge University Press, p. 2. LINKS Index entries for linear recurrences with constant coefficients, signature (0, -2, 0, -1). FORMULA a(2n) = (-1)^n*(2*n+1), a(2n+1) = (-1)^n*(n+1). Or (apart from signs and with offset 1), a(n) = n, n odd; n/2, n even. G.f.: (1+x-x^2)/(1+x^2)^2 - Len Smiley. a(-2-n) = (-1)^n * a(n). - Michael Somos, Jun 15 2005 a(0)=1, a(1)=1, a(2)=-3, a(3)=-2, a(n)=-2*a(n-2)-a(n-4) [From Harvey P. Dale, Dec 02 2011] a(n)=(-1)^floor(n/2)A026741(n+1). a(2*n) = A157142(n). a(2*n - 1) = A181983(n). - Michael Somos, Feb 22 2016 EXAMPLE G.f. = 1 + x - 3*x^2 - 2*x^3 + 5*x^4 + 3*x^5 - 7*x^6 - 4*x^7 + 8*x^9 + 5*x^10 + ... MATHEMATICA CoefficientList[Series[(1+x-x^2)/(1+x^2)^2, {x, 0, 60}], x] (* or *) LinearRecurrence[{0, -2, 0, -1}, {1, 1, -3, -2}, 70] a[ n_] := With[{m = n + 1}, m I^m / If[ Mod[ m, 2] == 1, I, -2]]; (* Michael Somos, Jun 11 2013 *) PROG (PARI) {a(n) = if( n==-1, 0, (-1)^(n\2) * (n+1) / gcd(n+1, 2))}; /* Michael Somos, Jun 15 2005 */ CROSSREFS Cf. A026741 is unsigned version. Cf. A157142, A181983. Sequence in context: A318516 A194748 A323462 * A176447 A145051 A026741 Adjacent sequences:  A030637 A030638 A030639 * A030641 A030642 A030643 KEYWORD sign,easy,nice AUTHOR STATUS approved

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Last modified September 18 10:31 EDT 2020. Contains 337166 sequences. (Running on oeis4.)