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a(n) = sigma(n) mod 9.
4

%I #19 Apr 25 2023 14:14:15

%S 1,3,4,7,6,3,8,6,4,0,3,1,5,6,6,4,0,3,2,6,5,0,6,6,4,6,4,2,3,0,5,0,3,0,

%T 3,1,2,6,2,0,6,6,8,3,6,0,3,7,3,3,0,8,0,3,0,3,8,0,6,6,8,6,5,1,3,0,5,0,

%U 6,0,0,6,2,6,7,5,6,6,8,6,4,0,3,8,0,6,3,0,0,0,4,6,2,0,3,0,8,0,3,1,3,0,5,3,3

%N a(n) = sigma(n) mod 9.

%C If gcd(m,n) = 1 then a(m*n) = (a(m) * a(n)) mod 9. - _Robert Israel_, Sep 14 2014

%H Robert Israel, <a href="/A105852/b105852.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = A010878(A000203(n)). - _Michel Marcus_, Sep 14 2014

%p seq(numtheory:-sigma(n) mod 9, n=1..1000); # _Robert Israel_, Sep 14 2014

%t Mod[DivisorSigma[1, Range[100]], 9] (* _Wesley Ivan Hurt_, Apr 25 2023 *)

%o (PARI) a(n)=sigma(n)%9

%Y Cf. A000203, A010878, A190998 (digital root of sigma(n)).

%K easy,nonn

%O 1,2

%A _Shyam Sunder Gupta_, May 05 2005

%E Name corrected and keyword base removed by _Michel Marcus_, Sep 14 2014