OFFSET
1,6
FORMULA
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
CROSSREFS
KEYWORD
nonn
AUTHOR
Brian Barsotti, Mar 04 2020
EXTENSIONS
Terms a(31) and beyond from Andrew Howroyd, Mar 05 2020
a(20) corrected by Georg Fischer, Oct 06 2024
STATUS
approved