factor.6

UNIX v6源代码 这几乎是最经典的unix版本 unix操作系统设计和莱昂氏unix源代码分析都是用的该版

.th FACTOR VI 1/15/73
.sh NAME
factor \*- discover prime factors of a number
.sh SYNOPSIS
.bd factor [ number ]
.sh DESCRIPTION
When
.it factor
is invoked without an argument, it waits for a number to be typed in.
If you type in a positive
number less than 2\u\s756\s0\d (about
.if n 7.2e16)
.if t 7.2\(mu10\u\s716\s0\d\|)
it will factor the number and print its prime factors; each one is printed
the proper number of times.
Then it waits for another number.
It exits if it encounters a zero or any non-numeric character.
.s3
If
.it factor
is invoked with an argument, it factors the number
as above and then exits.
.s3
Maximum time to factor is proportional to
.if n sqrt(n)
.if t \(sr\o'\fIn\fR\(rn'
and occurs when
.it n
is prime
or the square of a prime.
It takes 1 minute to factor a prime
near
10\u\s713\s0\d.
.sh DIAGNOSTICS
`Ouch.' for input out of range or for garbage input.
.sh BUGS