ZigNumber(n) = (-1)^n E_(2 n) for (n element Z and n>=0 and ZigNumber(0) = 1 and ZigNumber(1) = 1 and ZigNumber(2) = 5 and ZigNumber(3) = 61 and ZigNumber(4) = 1385 and ZigNumber(5) = 50521 and ...)