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 A221874 Numbers m such that 10*m^2 + 6 is a square. 8
 1, 5, 43, 191, 1633, 7253, 62011, 275423, 2354785, 10458821, 89419819, 397159775, 3395598337, 15081612629, 128943316987, 572704120127, 4896450447169, 21747674952197, 185936173675435, 825838944063359, 7060678149219361, 31360132199455445 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The Diophantine equation 10*x^2 + k = y^2, for |k| < 10, has integer solutions with the following k values: k = 1, the nonnegative x values are in A084070; k = -1, " A097315; k = 4, " 2*A084070; k = -4, " 2*A097315; k = 6, " this sequence; k = -6, " A221875; k = 9, " A075836; k = -9, " A052454. a(n+1)/a(n) tends alternately to (sqrt(2)+sqrt(5))^2/3 and (2*sqrt(2)+sqrt(5))^2/3; a(n+2)/a(n) tends to A176398^2. LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..1000 Index entries for linear recurrences with constant coefficients, signature (0,38,0,-1). FORMULA G.f.: x*(1+x)*(1+4*x+x^2)/((1-6*x-x^2)*(1+6*x-x^2)). a(n) = ((-5*(-1)^n+2*t)*(3+t)^(2*floor(n/2)) - (5*(-1)^n+2*t)*(3-t)^(2*floor(n/2)))/10, where t=sqrt(10). a(n) = 2*A129556(n) + 1. a(n)*a(n-3) - a(n-1)*a(n-2) = -36 + 12(-1)^n. MAPLE A221874:=proc(q) local n; for n from 1 to q do if type(sqrt(10*n^2+6), integer) then print(n); fi; od; end: A221874(100000000000000000); # Paolo P. Lava, Feb 11 2013 MATHEMATICA LinearRecurrence[{0, 38, 0, -1}, {1, 5, 43, 191}, 22] PROG (Magma) m:=22; R:=PowerSeriesRing(Integers(), m); Coefficients(R!((1+x)*(1+4*x+x^2)/((1-6*x-x^2)*(1+6*x-x^2)))); (Maxima) makelist(expand(((-5*(-1)^n+2*sqrt(10))*(3+sqrt(10))^(2*floor(n/2))-(5*(-1)^n+2*sqrt(10))*(3-sqrt(10))^(2*floor(n/2)))/10), n, 1, 22); CROSSREFS Cf. A097315, A084070, A075836, A052454, A129556, A221875. Subsequence of A031150. Sequence in context: A102851 A173554 A126963 * A317282 A182191 A038140 Adjacent sequences: A221871 A221872 A221873 * A221875 A221876 A221877 KEYWORD nonn,easy AUTHOR Bruno Berselli, Jan 28 2013 STATUS approved

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