UPDATE: corrected direction of hysteresis. Also cut down resistances to reduce parasitic capacitance impacts.
Can be used from shifting 2V input to 5V output, 5V input to 2V output, and anything in between.
Hysteresis is about 5% of Vout; for a 5V output, this means approximately 250mV hysteresis. If not convinced, use superposition: dVin-/dVout= (10k//10k)/[10k//10k+100k]=5k/105K=4.76%.
That hysteresis keeps the circuit from double pulsing for slow or non-monotonic edges on the input; this is critical for fast digital circuits. It is also nice to have an output logic level that can be completely independent from the input logic level.
