IPA: /əˈrɪθməˌtɪʃən/
KK: /əˈrɪθməˌtɪʃən/
A person who is skilled in arithmetic, which is the branch of mathematics dealing with numbers and basic operations like addition, subtraction, multiplication, and division.
The arithmetician solved the complex math problem quickly.
Arithmetician → It is formed from "arithmetica" (from Greek "arithmetike", meaning the art of counting) and "-ian" (meaning a person who practices or is concerned with). The word refers to a person who practices or is skilled in the art of counting or mathematics.
Think of someone who is skilled in the 'art of counting' — that's what an arithmetician does.