, , , . , , , .
, , . , , d N, t t +d t, d t N t: (256.1) l , ; , . : (256.2) N 0 ( t =0), N t. (256.2) , .
: 1/2 t . 1/2 , . , (256.2),
- .
d N t|dN|=lNt d t. t (. . 0 ¥) N 0, t :
( (256.2)). , t , l ( , Z N) , 1 : (256.3)
(): 1 , 1 . (): 1 = 3,7×1010.
, , . : (256.4)
(256.5) , Y , (a -), ( 1, ).