a(n) is the number of sets modulo n which can be formed by a finite arithmetic sequence, whose complement cannot be formed by a finite arithmetic sequence.
a(n) = n*(sigma(n) - tau(n) - n + (n mod 2)) for n > 2.
a(p) = 0 for all primes p.
EXAMPLE
One example of such a set would be {0, 2, 4} mod 8. This set can be formed by starting with 0 and adding 2 twice. However, the set's complement, {1, 3, 5, 6, 7} mod 8, cannot be formed by any arithmetic sequence without including the original set.
PROG
(PARI) a(n)={if(n<=2, 0, n*(sigma(n) - numdiv(n) - n + n%2))} \\ Andrew Howroyd, Mar 05 2020
