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A161462
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Final digit of sum of all the natural numbers from n to 2*n-1.
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0
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1, 5, 2, 2, 5, 1, 0, 2, 7, 5, 6, 0, 7, 7, 0, 6, 5, 7, 2, 0, 1, 5, 2, 2, 5, 1, 0, 2, 7, 5, 6, 0, 7, 7, 0, 6, 5, 7, 2, 0, 1, 5, 2, 2, 5, 1, 0, 2, 7, 5, 6, 0, 7, 7, 0, 6, 5, 7, 2, 0, 1, 5, 2, 2, 5, 1, 0, 2, 7, 5, 6, 0, 7, 7, 0, 6, 5, 7, 2, 0, 1, 5, 2, 2, 5, 1
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OFFSET
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1,2
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COMMENTS
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Periodic with period length 20. - R. J. Mathar, May 21 2010
Also, units digits of the pentagonal numbers. - Ant King, Jul 12 2011
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LINKS
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Table of n, a(n) for n=1..86.
Index to sequences with 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) = A000326(n) mod 10. a(n)= +a(n-5) -a(n-10) +a(n-15). [From R. J. Mathar, May 21 2010]
a(n)=a(n-20) (Ant King, Jul 12 2011)
a(n)=70-a(n-1)-a(n-2)-a(n-3)-....-a(n-19) [Ant King, Jul 12 2011]
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EXAMPLE
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If n=1, then a(1)=1. If n=2 and 2+3=5, then a(2)=5. if n=3 and 3+4+5=12, then a(3)=2, etc.
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MATHEMATICA
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Table[Mod[n*(3*n - 1)/2, 10], {n, 100}] (* Ant King, Jul 12 2011 *)
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CROSSREFS
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Cf. A000027, A000326.
Sequence in context: A093008 A198496 A177925 * A164557 A217868 A153842
Adjacent sequences: A161459 A161460 A161461 * A161463 A161464 A161465
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KEYWORD
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nonn,base,easy
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AUTHOR
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Juri-Stepan Gerasimov, Jun 10 2009
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EXTENSIONS
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Entries confirmed by R. J. Mathar, May 21 2010
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STATUS
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approved
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