1,1

a(n) is numbers k such that antiharmonic mean of divisors of k is not an integer. Antiharmonic mean of divisors of number m = Product (p_i^e_i) is A001157(m)/A000203(m) = Product ((p_i^(e_i+1)+1)/(p_i+1)). a(n) = k = A001157(k)/A000203(k) for A001157(k)/A000203(k)is not integer. a(n)non-occurring in A020487.

a(12) = 15, sigma_2(15)/sigma_1(15)=260/24 = 65/6 (not integer).

Cf. A001157, A000203, A020487.

Sequence in context: A028797 A028773 A028813 this_sequence A028793 A028753 A028745

Adjacent sequences: A158273 A158274 A158275 this_sequence A158277 A158278 A158279

nonn

Jaroslav Krizek (jaroslav.krizek(AT)atlas.cz), Mar 15 2009