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A213472
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Period 20, repeat [1, 4, 0, 9, 1, 6, 4, 5, 9, 6, 6, 9, 5, 4, 6, 1, 9, 0, 4, 1].
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0
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1, 4, 0, 9, 1, 6, 4, 5, 9, 6, 6, 9, 5, 4, 6, 1, 9, 0, 4, 1, 1, 4, 0, 9, 1, 6, 4, 5, 9, 6, 6, 9, 5, 4, 6, 1, 9, 0, 4, 1, 1, 4, 0, 9, 1, 6, 4, 5, 9, 6, 6, 9, 5, 4, 6, 1, 9, 0, 4, 1, 1, 4, 0, 9, 1, 6, 4, 5, 9, 6, 6, 9, 5, 4, 6, 1, 9, 0, 4, 1, 1, 4, 0, 9, 1, 6
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OFFSET
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0,2
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COMMENTS
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Units digits of the centered triangular numbers A005448(n).
The cyclic part of this sequence is palindromic.
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LINKS
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Index entries for linear recurrences with constant coefficients, signature (0,0,0,0,1,0,0,0,0,-1,0,0,0,0,1).
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FORMULA
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a(n) = a(n-5)-a(n-10)+a(n-15).
a(n) = a(n-20).
a(n) = 45-a(n-1)-a(n-2)-a(n-3)-a(n-4)-a(n-10)-a(n-11)-a(n-12)-a(n-13)-a(n-14).
a(n) = 90 - Sum_{i=1..19} a(n-i), with n > 19.
a(n) = (3n^2/2+3n/2+1) mod 10.
G.f.: (1+x+x^2)*(1+3*x-4*x^2+10*x^3-5*x^4+5*x^6-5*x^8+10*x^9-4*x^10+3*x^11+x^12) / ((1-x)*(1+x^2)*(1+x+x^2+x^3+x^4)*(1-x^2+x^4-x^6+x^8)). - Bruno Berselli, Jun 13 2012
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EXAMPLE
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As the seventh centered triangular number is A005448(7)=64, which has units’ digit 4, then a(7)=4
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MATHEMATICA
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Mod[1/2(3#^2-3#+2), 10] &/@Range[86]
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PROG
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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