def _igam(a, x):
<EOL>ax = math.exp(a * math.log(x) - x - math.lgamma(a))<EOL>r = a<EOL>c = <NUM_LIT:1.0><EOL>ans = <NUM_LIT:1.0><EOL>while True:<EOL><INDENT>r += <NUM_LIT:1.0><EOL>c *= x/r<EOL>ans += c<EOL>if c / ans <= MACHEP:<EOL><INDENT>return ans * ax / a<EOL><DEDENT><DEDENT>
Left tail of incomplete Gamma function:

            inf.      k
     a  -x   -       x
    x  e     >   ----------
             -     -
            k=0   | (a+k+1)