proposed
editing
The cubefree kernel of 56=7*2^3 is 28=7*2^2 and the divisors <= 28 of 56 are {1, 2, 4, 7, 8, 14, 28}, therefore a(56)=7.
Cf. A000005, A073183, A073180.
approved
reviewed
Michel Marcus: A073183 too ?