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 A070199 Number of palindromes of length <= n. 4
 10, 19, 109, 199, 1099, 1999, 10999, 19999, 109999, 199999, 1099999, 1999999, 10999999, 19999999, 109999999, 199999999, 1099999999, 1999999999, 10999999999, 19999999999, 109999999999, 199999999999, 1099999999999, 1999999999999, 10999999999999, 19999999999999 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Colin Barker, Table of n, a(n) for n = 1..1000 Index entries for linear recurrences with constant coefficients, signature (1,10,-10). FORMULA From Colin Barker, Jun 30 2012: (Start) a(n) = a(n-1) + 10*a(n-2) - 10*a(n-3). G.f.: x*(10 + 9*x - 10*x^2)/((1 - x)*(1 - 10*x^2)). (End) a(n)=(-2*Sqrt[10]+10^(n/2)*(11+2*Sqrt[10]+(-1)^n*(-11+2*Sqrt[10])))/ (2*Sqrt[10]). - Harvey P. Dale, Mar 18 2016 From Colin Barker, Mar 17 2017: (Start) a(n) = 2^(n/2+1)*5^(n/2) - 1 for n even. a(n) = 11*10^((n-1)/2) - 1 for n odd. (End) MATHEMATICA LinearRecurrence[{1, 10, -10}, {10, 19, 109}, 30] (* Harvey P. Dale, Mar 18 2016 *) PROG (PARI) Vec(x*(10+9*x-10*x^2)/((1-x)*(1-10*x^2)) + O(x^40)) \\ Colin Barker, Mar 17 2017 CROSSREFS Partial sums of A070252. Sequence in context: A166706 A131495 A060630 * A015445 A220005 A253213 Adjacent sequences:  A070196 A070197 A070198 * A070200 A070201 A070202 KEYWORD nonn,base,easy AUTHOR N. J. A. Sloane and Robert G. Wilson v, May 14 2002 STATUS approved

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