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A289120
a(n) is the number of odd integers divisible by 7 in ]2*(n-1)^2, 2*n^2[.
6
0, 0, 1, 0, 1, 2, 1, 2, 2, 3, 2, 3, 4, 3, 4, 4, 5, 4, 5, 6, 5, 6, 6, 7, 6, 7, 8, 7, 8, 8, 9, 8, 9, 10, 9, 10, 10, 11, 10, 11, 12, 11, 12, 12, 13, 12, 13, 14, 13, 14, 14, 15, 14, 15, 16, 15, 16, 16, 17, 16, 17, 18, 17, 18, 18, 19, 18, 19, 20, 19, 20, 20, 21, 20, 21, 22, 21, 22, 22, 23, 22, 23, 24, 23, 24, 24, 25, 24, 25, 26, 25, 26, 26, 27, 26, 27, 28, 27, 28, 28, 29
OFFSET
0,6
COMMENTS
This sequence has the form (0+2k,0+2k,1+2k,0+2k,1+2k,2+2k,1+2k) for k>=0.
FORMULA
From Colin Barker, Jul 02 2017: (Start)
G.f.: x^2*(1 + x)*(1 - x + x^2)^2 / ((1 - x)^2*(1 + x + x^2 + x^3 + x^4 + x^5 + x^6)).
a(n) = a(n-1) + a(n-7) - a(n-8) for n>7.
(End)
MATHEMATICA
Table[Count[Mod[Table[2 ((n - 1)^2 + k) - 1, {k, 1, 2 n - 1}], 7],
0], {n, 0, 100}]
PROG
(PARI) concat(vector(2), Vec(x^2*(1 + x)*(1 - x + x^2)^2 / ((1 - x)^2*(1 + x + x^2 + x^3 + x^4 + x^5 + x^6)) + O(x^100))) \\ Colin Barker, Jul 02 2017
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Ralf Steiner, Jun 25 2017
STATUS
approved