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组合comb
排列perm
n！fact

comb(n,r)
Returns the number of combinations of n elements taken r at
a time (0 ≤  r ≤ n). Use comb to determine the total possible
number of groups for a given number of items. A combina-
tion is any set or subset of items, regardless of their order.
Combinations are distinct from permutations, for which the
order is significant. comb(n,r) also is known as the binomial
coefficient and is read “n choose r”. The number of combina-
tions is n!/(r!(n–r)!) where n and r are integers and the symbol
! denotes a factorial.  comb(n,r) is the same as  fact(n)/
(fact(r)*(fact(n-r)) . See also: fact (p. 66), perm (p. 115).
Examples:
comb(8,0) → 1.
comb(8,1) → 8.
comb(8,2) → 28.
comb(8,6) → 28.
comb(8,8) → 1.

perm(n[,r])
Returns the number of permutations of n elements taken r at
a time (0 ≤ r ≤ n). If r is omitted, perm(n) returns the factorial
of n. A permutation is any set or subset of items where order
is significant. Permutations are distinct from combinations,
for which order doesn’t matter. The number of permutations
is  n!/(n–r)! where  n and  r are integers and the symbol !
denotes the factorial.  perm(n,r) is the same as  fact(n)/
fact(n-r). See also: comb (p. 45), fact (p. 66).
Examples:
perm(8,0) → 1.
perm(8,1) → 8.
perm(8,2) → 56.
perm(8,6) → 20160.
perm(8,8) → 40320.
perm(8) → 40320.

fact(n)
Returns the factorial of n (≥ 0), given by n! = 1 × 2 ×  ⋅⋅⋅ × n,
where n is an integer. The special case 0! is defined to be equal
to 1. Note that fact(n) equals gamma(n+1).
Examples:
fact(0) → 1.
fact(1) → 1.
fact(8) → 40320.
fact(-8) → . (missing—n < 0).
fact(8.5) → . (missing—noninteger n).
fact(int(8.5)) → 40320.
fact(20) → 2.432902E18.