OFFSET
1,3
COMMENTS
Sum of all odd prime divisors of all positive integers <= n.
FORMULA
EXAMPLE
For n = 7 the odd prime divisors of the first seven positive integers are {0}, {0}, {3}, {0}, {5}, {3}, {7} so a(7) = 0 + 0 + 3 + 0 + 5 + 3 + 7 = 18.
MATHEMATICA
Table[Sum[Prime[k + 1] Floor[n/Prime[k + 1]], {k, 1, n}], {n, 70}]
Rest[nmax = 70; CoefficientList[Series[(1/(1 - x)) Sum[Prime[k] x^Prime[k]/(1 - x^Prime[k]), {k, 2, nmax}], {x, 0, nmax}], x]]
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Ilya Gutkovskiy, Dec 24 2016
STATUS
approved