以前的数论教材不是都把0算作非奇非偶数（什么玩意？:D ）
现在有种说法又承认0是偶数
比如wiki的说法是承认0是偶数的
This concept begins with integers. An even number is an integer that is "evenly divisible" by 2, i.e., divisible by 2 without remainder; an odd number is an integer that is not evenly divisible by 2. (The old-fashioned term "evenly divisible" is now almost always shortened to "divisible".) A formal definition of an odd number is that it is an integer of the form n = 2k + 1, where k is an integer. An even number has the form n = 2k where k is an integer.
Examples of even numbers are −4, 8, 0, and 42. Examples of odd numbers are −3, 9, 1, and 5. A fractional number like 1/2 or 3.141 is neither even nor odd.
The set of even numbers can be written:
{Evens} = 2Z = {..., −6, −4, −2, 0, 2, 4, 6, ...},
where Z is the set of all integers. The set of odd numbers can be shown like this:
{Odds} = 2Z + 1 = {..., −5, −3, −1, 1, 3, 5, ...}.
