A165186 - OEIS

%I #4 Oct 15 2021 14:32:31

%S 0,1,4,6,10,17,28,36,30,45,66,82,78,105,140,136,136,141,190,230,238,

%T 253,322,380,250,325,360,434,406,505,558,592,572,561,700,678,666,741,

%U 910,980,820,917,946,1122,1050,1173,1316,1432,1078,1125,1394,1430,1378,1449

%N a(n) = Sum_{k=1..n} (k*(n-k) mod n).

%C Comment from Max Alekseyev, Nov 22 2009: For a prime p==3 (mod 4), a(p) = p*h(-p) + p*(p-1)/2 where h(-p) is the class number (listed in A002143). For example, h(-19)=1 and a(19) = 19*1 + 19*18/2 = 190.

%t Table[Sum[Mod[k (n-k),n],{k,n}],{n,100}]

%Y Cf. A008784, A048153

%K nonn

%O 1,3

%A _Wouter Meeussen_, Sep 06 2009