, n .
b : ab b. a b. , 2 = 2.
, b, ab, b ab:
cab, acb, abc.
b :
cba, bca, bac.
, 2 3 = 6 , 6. P3 = 2 · 3 = 6.
k , k . ³ : 123...k. (k + 1)- . :
1) 1;
2) 2;
3) a 3;
k) k - k;
(k + 1) , , k + 1 .
, k + 1 (k + 1) , k , ,
.
,
P1 = 1;
P2 = P1 2 = 1 2 = 2;
P3 = P2 3 = 1 2 3 = 6;
P4 = 4 = 1 2 3 4 = 24;
P5 = P4 5 = 1 ∙ 2 ∙ 3 ∙ 4 ∙ 5 = 120;
Pk = Pk-1 k = 1-2 3 · ... · k;
Pk+1 = Pk (k+ 1 ) = 1 2 3 ... k (k +l).
1 η n n! 1 10.
n 1 , ! (: ).
. 6 ?
'
P6 = 6! =l 2 3 4 5 6 = 720.