OFFSET
1,1
COMMENTS
a(n) is the index of the first position of prime(n) in A027750, the sequence that lists the divisors of all integers. - Michel Marcus, Oct 17 2015
LINKS
Chai Wah Wu, Table of n, a(n) for n = 1..10000
EXAMPLE
For n = 3 the third prime number is 5 and the sum of the number of divisors of the first five positive integers is 1 + 2 + 2 + 3 + 2 = 10, so a(3) = 10.
PROG
(PARI) a(n) = sum(i=1, prime(n), numdiv(i)); \\ Michel Marcus, Jan 15 2015
(Python)
from math import isqrt
from sympy import prime
def A253769(n): return (lambda m, p: 2*sum(p//k for k in range(1, m+1))-m*m)(isqrt(prime(n)), prime(n)) # Chai Wah Wu, Oct 09 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
Omar E. Pol, Jan 14 2015
STATUS
approved