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 A209291 Sum of the refactorable numbers less than or equal to n. 0

%I

%S 1,3,3,3,3,3,3,11,20,20,20,32,32,32,32,32,32,50,50,50,50,50,50,74,74,

%T 74,74,74,74,74,74,74,74,74,74,110,110,110,110,150,150,150,150,150,

%U 150,150,150,150,150,150,150,150,150,150,150,206,206,206,206,266

%N Sum of the refactorable numbers less than or equal to n.

%C A number is refactorable if it is divisible by the number of its divisors.

%C The first 8 terms of a(n) are odd. The next odd term after 11 is a(225) = 2395.

%F a(n) = sum( (i) * ( 1 + floor(i/d(i)) - ceil(i/d(i)) ), i = 1..n ), where d(n) is the number of divisors less than or equal to n.

%p with(numtheory); a:=n->sum( (i) * ( 1 + floor(i/tau(i)) - ceil(i/tau(i)) ), i = 1..n );

%Y Cf. A033950, A000005, A141586, A057265, A036896, A036898, A114617.

%K nonn

%O 1,2

%A _Wesley Ivan Hurt_, Jan 16 2013

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Last modified July 15 20:24 EDT 2019. Contains 325056 sequences. (Running on oeis4.)