 Subadditivity of the base-p digit sum

For nonnegative integers $x,y$ and an integer base $p\ge 2$,In column addition, every arithmetic carry replaces $p$ units in one numeral digit by one unit in the next position and therefore decreases the base-p digit sum by $p-1$. Count each outgoing carry separately, including each step of a chain. There is equality precisely when there are no carries. For prime number bases, Kummer's theorem turns this digit-sum deficit into a prime exponent.