This is a (inverting) Schmitt trigger. Resistance R1 can be chosen, resistance R2 is defined by the desired hysteresis. The output gets Vn for input voltage > 0 + vHys and Vp for input voltage < vHys*Vns/Vps.
The example is taken from: U. Tietze and C. Schenk, Halbleiter-Schaltungstechnik (German), 11th edition, Springer 1999, Chapter 6.5.2