OFFSET
1,2
COMMENTS
a(n) has a symmetric representation.
FORMULA
a(n) = A048050(2*n).
Sum_{k=1..n} a(k) = (5*Pi^2/24 - 1) * n^2 + O(n*log(n)). - Amiram Eldar, Mar 21 2024
EXAMPLE
For n = 5 the 5th even number is 10 and the divisors of 10 are [1, 2, 5, 10] and the sum of the divisors of 10 except the smaller and the largest is 2 + 5 = 7, so a(5) = 7.
MATHEMATICA
a[n_] := DivisorSigma[1, 2*n] - 2*n - 1; Array[a, 100] (* Amiram Eldar, Aug 19 2021 *)
PROG
(Python)
from sympy import divisors
def a(n): return sum(divisors(2*n)[1:-1])
print([a(n) for n in range(1, 66)]) # Michael S. Branicky, Aug 19 2021
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Omar E. Pol, Aug 18 2021
STATUS
approved