# Sum of digits in base $b$ the $n^{th}$ row of Pascal's Triangle

Number of digits in row of Pascal's triangle is $O\left(n^2\right)$

Out of curiosity, continuing off of the linked question, I present the following question:

What is the asymptotic growth of the sum of all the digits in the $n^{\text{th}}$ row of Pascal's Triangle, when all the binomial coefficients are written in base $b$?

It can be assumed that $b$ is a positive integer.

You can assume that most of the digits are random, so if there are $m$ digits in the sum, the sum of digits is about $\frac {b-1}2m$. The leading digits may be biased low, but there are only $n$ leading digits in row $n$, so the non-leading digits will dominate. The sum of digits is then $\frac {b-1}2 n^2 \in O( n^2)$
• In that case, why isn't the sum of digits asymptotic to $n^2$, just like the number of digits? – Jack Lam Aug 29 '16 at 10:01
• I had confused myself into taking the number of digits in $n^2$. Fixed – Ross Millikan Aug 29 '16 at 14:52