Algebraic Number Theory
《Algebraic Number Theory》 · 第 200 页 · Section 7.1「Numbers composed of small primes」
The function $\rho(u)$ that arises here is known as the Dickman function; it may be defined to be the unique continuous function on $[0,\infty)$ satisfying the differential–delay equation $u\rho'(u)=-\rho(u-1)$ for $u>1$ together with the initial condition $\rho(u)=1$ for $0\le u\le1$.