It is also a Motzkin number,  a Bell number, an all-Harshad number, a meandric number, a semi-meandric number, an open meandric number, and the third (or fourth) Fibonacci number.An integer is called even if it is divisible by 2. For integers written in a numeral system based on an even number such as decimal, divisibility by 2 is easily tested by merely looking at the last digit. If it is even, then the whole number is even. In particular, when written in the decimal system, all multiples of 2 will end in 0, 2, 4, 6, or 8.Two is the base of the binary system, the numeral system with the fewest tokens that allows denoting a natural number n substantially more concisely (with log2 n  tokens) than a direct representation by the corresponding count of a single token (with n tokens). This binary number system is used extensively in computing. For any number x:

x + x = 2 · x addition to multiplication
x · x = x2 multiplication to exponentiation
xx = x↑↑2 exponentiation to tetrationExtending this sequence of operations by introducing the notion of hyperoperations, here denoted by "hyper(a,b,c)" with a and c being the first and second operand, and b being the level in the above sketched sequence of operations, the following holds in general:

hyper(x,n,x) = hyper(x,(n + 1),2).Two has therefore the unique property that 2 + 2 = 2 · 2 = 22 = 2↑↑2 = 2↑↑↑2 = ..., disregarding the level of the hyperoperation, here denoted by Knuths up-arrow notation.
From 1995 to 1998, the U.S. SIDS rate for births at 37–39 weeks of gestation was 0.73/1000, while the SIDS rate for births at 28–31 weeks of gestation was 2.39/1000.Anemia has also been linked to SIDS (however, per item 6 in the list of epidemiologic characteristics below, extent of anemia cannot be evaluated at autopsy because an infants total hemoglobin can only be measured during life). SIDS incidence rises from zero at birth, is highest from two to four months of age, and declines toward zero after the infants first year. Genetics
Genetics plays a role, as SIDS is more prevalent in males. There is a consistent 50% male excess in SIDS per 1000 live births of each sex. Given a 5% male excess birth rate, there appears to be 3.15 male SIDS cases per 2 female cases, for a male fraction of 0.61. This value of 61% in the US is an average of 57% black male SIDS, 62.2% white male SIDS and 59.4% for all other races combined. Note that when multiracial parentage is involved, infant race is arbitrarily assigned to one category or the other; most often it is chosen by the mother. The X-linkage hypothesis for SIDS and the male excess in infant mortality have shown that the 50% male excess might be related to a dominant X-linked allele, occurring with a frequency of 1⁄3 that is protective against transient cerebral anoxia. An unprotected male would occur with a frequency of 2⁄3 and an unprotected female would occur with a frequency of 4⁄9.