Determining output impedance for a pulse generator
The output stage of the lm290x series comparator is an open collector bjt.


R6 and R7 are simply acting as a voltage divider. If R6 = R7 the voltage at the mid point will be half the supply (i.e. = 6V) regardless of the actual value chosen between 1k and 100k. 

Over this range of resistance the current through the output transistor will of change, this has the effect of changing the saturation voltage across the transistor (typically 100mV). 

The data sheet (http://www.diodes.com/_files/datasheets/LM2901_03.pdf) suggests a range of 6mA minimum with 16mA typical. This minimum value would be achieved with R6= R7 = 1k0.