440220domain: NAppears in sequences(-1)sigma sociable number of order 2: (-1)sigma((-1)sigma(x))=x, but (-1)sigma(x)<>x, where if x=Product p(i)^r(i) then (-1)sigma(x)=Product (-1+Sum p(i)^s(i), s(i)=1 to r(i)); (-1)sigma(1)=1.at n=7A051152a(n) = period length of the sequence A020639(n^k - 1), k >= 1.at n=69A368811