OFFSET
1,2
FORMULA
a(n) = Sum_{d1|n, d2|n, d1<=d2} c(d1 + d2), where c is the prime characteristic (A010051).
EXAMPLE
a(12) = 7; The pairs are (1,1), (1,2), (1,4), (1,6), (1,12), (2,3), (3,4).
CROSSREFS
KEYWORD
nonn
AUTHOR
Wesley Ivan Hurt, Feb 14 2022
STATUS
approved