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 A220082 Numbers k such that 10*k-1 is a square. 4
 1, 5, 17, 29, 53, 73, 109, 137, 185, 221, 281, 325, 397, 449, 533, 593, 689, 757, 865, 941, 1061, 1145, 1277, 1369, 1513, 1613, 1769, 1877, 2045, 2161, 2341, 2465, 2657, 2789, 2993, 3133, 3349, 3497, 3725, 3881, 4121, 4285, 4537, 4709, 4973, 5153, 5429, 5617, 5905 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Equivalently, numbers of the form m*(10*m+6)+1, where m=0,-1,1,-2,2,-3,3,... LINKS Bruno Berselli, Table of n, a(n) for n = 1..1000 Index entries for linear recurrences with constant coefficients, signature (1,2,-2,-1,1). FORMULA G.f.: x*(1+4*x+10*x^2+4*x^3+x^4)/((1+x)^2*(1-x)^3). a(n) = a(-n+1) = (10*n*(n-1)-(2*n-1)*(-1)^n+3)/4. For the definition: 10*a(n)-1 = ((10*n-(-1)^n-5)/2)^2. a(n) = A212570(n)-A212570(n-1) = 4*A085787(n-1)+1 = A132356(n-1)-(2*n-1)*(-1)^n. MAPLE A220082:=proc(q) local n; for n from 1 to q do if type(sqrt(10*n-1), integer) then print(n); fi; od; end: A220082(1000); # Paolo P. Lava, Feb 19 2013 MATHEMATICA Select[Range[0, 6000], IntegerQ[Sqrt[10 # - 1]] &] CoefficientList[Series[(1 + 4 x + 10 x^2 + 4 x^3 + x^4) / ((1 + x)^2 (1 - x)^3), {x, 0, 50}], x] (* Vincenzo Librandi, Aug 18 2013 *) PROG (MAGMA) [n: n in [1..6000] | IsSquare(10*n-1)]; /* or (see the first comment): */ [1] cat [m*(10*m+6)+1: m in [-n, n], n in [1..24]]; (MAGMA) I:=[1, 5, 17, 29, 53]; [n le 5 select I[n] else Self(n-1) +2*Self(n-2)-2*Self(n-3)-Self(n-4)+Self(n-5): n in [1..60]]; // Vincenzo Librandi, Aug 18 2013 CROSSREFS Cf. A085787, A132356 (numbers n such that 10*n+1 is a square). Cf. numbers n such that k*n-1 is a square: A002522 (k=1), A001844 (k=2), A062317 (k=5). Sequence in context: A024351 A246325 A086523 * A207337 A145478 A143103 Adjacent sequences:  A220079 A220080 A220081 * A220083 A220084 A220085 KEYWORD nonn,easy AUTHOR Bruno Berselli, Dec 05 2012 STATUS approved

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Last modified January 26 11:07 EST 2021. Contains 340438 sequences. (Running on oeis4.)