subscript sign in math environment

[Wolfgang Engelmann]

How do i write in math mode

quantauxin=QANTAUXIN*(1+DIUR_X*sin(v_DIUR_LAGE)*2PI)*e^{-v/(DIUR_H*ln2)} 

i.e it should stay in the output as v_DIUR_LAGE ...

and not change v_DIUR  to v subscript DIUR ...