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 A008497 a(n) = floor(n/5)*floor((n+1)/5). 1
 0, 0, 0, 0, 0, 1, 1, 1, 1, 2, 4, 4, 4, 4, 6, 9, 9, 9, 9, 12, 16, 16, 16, 16, 20, 25, 25, 25, 25, 30, 36, 36, 36, 36, 42, 49, 49, 49, 49, 56, 64, 64, 64, 64, 72, 81, 81, 81, 81, 90, 100, 100, 100, 100, 110, 121, 121, 121, 121 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,10 LINKS Index entries for linear recurrences with constant coefficients, signature (1,0,0,0,2,-2,0,0,0,-1,1). FORMULA From R. J. Mathar, Apr 16 2010: (Start) a(n) = A002266(n)*A002266(n+1). a(n)= a(n-1) + 2*a(n-5) - 2*a(n-6) - a(n-10) + a(n-11). G.f.: x^5*(1+x^4)/ ((x^4+x^3+x^2+x+1)^2 * (1-x)^3). (End) MATHEMATICA Times@@@Partition[Floor[Range[0, 60]/5], 2, 1] (* or *) LinearRecurrence[ {1, 0, 0, 0, 2, -2, 0, 0, 0, -1, 1}, {0, 0, 0, 0, 0, 1, 1, 1, 1, 2, 4}, 60] (* Harvey P. Dale, Feb 01 2015 *) PROG (PARI) a(n) = (n\5)*((n+1)\5); \\ Michel Marcus, Jan 06 2017 CROSSREFS Cf. A002266. Sequence in context: A160409 A035645 A063440 * A220497 A194443 A220523 Adjacent sequences:  A008494 A008495 A008496 * A008498 A008499 A008500 KEYWORD nonn AUTHOR STATUS approved

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Last modified June 26 20:24 EDT 2019. Contains 324380 sequences. (Running on oeis4.)