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 A280181 Indices of centered 9-gonal numbers (A060544) that are also squares (A000290). 1
 1, 17, 561, 19041, 646817, 21972721, 746425681, 25356500417, 861374588481, 29261379507921, 994025528680817, 33767606595639841, 1147104598723073761, 38967788749988868017, 1323757712900898438801, 44968794449880558051201, 1527615253583038075302017 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also positive integers y in the solutions to 2*x^2 - 9*y^2 + 9*y - 2 = 0, the corresponding values of x being A046176. Consider all ordered triples of consecutive integers (k, k+1, k+2) such that k is a square and k+1 is twice a square; then the values of k are the squares of the NSW numbers (A002315), the values of k+1 are twice the squares of the odd Pell numbers (A001653), and the values of k+2 are thrice the terms of this sequence. (See the Example section.) - Jon E. Schoenfield, Sep 06 2019 LINKS Colin Barker, Table of n, a(n) for n = 1..650 Index entries for linear recurrences with constant coefficients, signature (35,-35,1). FORMULA a(n) = (6 + (3-2*sqrt(2))*(17+12*sqrt(2))^(-n) + (3+2*sqrt(2))*(17+12*sqrt(2))^n) / 12. a(n) = 35*a(n-1) - 35*a(n-2) + a(n-3) for n>3. G.f.: x*(1 - 18*x + x^2) / ((1 - x)*(1 - 34*x + x^2)). a(n) = (A002315(n-1)^2 + 2)/3 = (2*A001653(n)^2 + 1)/3. - Jon E. Schoenfield, Sep 06 2019 a(n) = A077420(floor((n-1)/2)) * A056771(floor(n/2)). - Jon E. Schoenfield, Sep 08 2019 E.g.f.: -1+(1/12)*(6*exp(x)+(3-2*sqrt(2))*exp((17-12*sqrt(2))*x)+(3+2*sqrt(2))*exp((17+12*sqrt(2))*x)). - Stefano Spezia, Sep 08 2019 EXAMPLE 17 is in the sequence because the 17th centered 9-gonal number is 1225, which is also the 35th square. From Jon E. Schoenfield, Sep 06 2019: (Start) The following table illustrates the relationship between the NSW numbers (A002315), the odd Pell numbers (A001653), and the terms of this sequence: .   |  A002315(n-1)^2  |   2*A001653(n)^2  | n |   = 3*a(n) - 2   |    = 3*a(n) - 1   |       3*a(n) --+------------------+-------------------+------------------- 1 |    1^2 =       1 |   1^2*2 =       2 |      1*3 =       3 2 |    7^2 =      49 |   5^2*2 =      50 |     17*3 =      51 3 |   41^2 =    1681 |  29^2*2 =    1682 |    561*3 =    1683 4 |  239^2 =   57121 | 169^2*2 =   57122 |  19041*3 =   57123 5 | 1393^2 = 1940449 | 985^2*2 = 1940450 | 646817*3 = 1940451 (End) MATHEMATICA LinearRecurrence[{35, -35, 1}, {1, 17, 561}, 50] (* G. C. Greubel, Dec 28 2016 *) PROG (PARI) Vec(x*(1 - 18*x + x^2) / ((1 - x)*(1 - 34*x + x^2)) + O(x^20)) CROSSREFS Cf. A000290, A001653, A002315, A046176, A046177, A056771, A060544, A077420. Sequence in context: A112716 A218351 A197395 * A012069 A191865 A249862 Adjacent sequences:  A280178 A280179 A280180 * A280182 A280183 A280184 KEYWORD nonn,easy AUTHOR Colin Barker, Dec 28 2016 STATUS approved

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Last modified July 6 14:10 EDT 2022. Contains 355110 sequences. (Running on oeis4.)