

A166925


Digital root of square of nth triangular number.


1



1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9
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OFFSET

1,2


COMMENTS

Period is (1,9,9).


LINKS

Table of n, a(n) for n=1..87.
Index entries for linear recurrences with constant coefficients, signature (0,0,1).


FORMULA

a(n)=(1/9)*{43*(n mod 3)+19*[(n+1) mod 3]5*[(n+2) mod 3]}, with n>=1 [From Paolo P. Lava, Nov 02 2009]
G.f.: x*(1+9*x+9*x^2)/(1x^3)  Bruno Berselli, May 05 2011


MATHEMATICA

Table[If[(m9 = Mod[(n(n + 1))^2/4, 9]) == 0, 9, m9], {n, 200}]


PROG

(MAGMA) &cat[[1, 9, 9]: n in [1..29]]; // Bruno Berselli, May 05 2011


CROSSREFS

Cf. A000217, A000537, A010888.
Sequence in context: A145280 A144667 A118428 * A178164 A216035 A171487
Adjacent sequences: A166922 A166923 A166924 * A166926 A166927 A166928


KEYWORD

base,less,easy,nonn


AUTHOR

Zak Seidov, Oct 23 2009


STATUS

approved



