There are k kinds of postcards, each with a limited amount. Let there be ai copies of the i‐th postcard. How many ways are there to send these postcards to n friends?
There are k kinds of postcards, each with a limited amount. Let there be ai copies of
the i‐th postcard. How many ways are there to send these postcards to n friends?
(Each friend can receive zero cards, as well as multiple copies of the same card.)