P-adic values

Given a prime number p the p-adic value is the function, denoted $v_{p}$ which takes as its argument a natural number n and returns the power of p appearing in the prime factorization of that number (equivalently, the highest power of p which divides n): $v_{p}(x)=\max\{n:p^{n}\mid x\}$. For example, the p-adic values of 60 for p=2,3,5,7,11,13... are 2,1,1,0,0,0,....

By convention, $v_{p}(0)=\infty$ for all primes p.

Here are some important properties of p-adic values:

• p-adic values convert multiplication into addition (akin to the logarithm function): $v_{p}(xy)=v_{p}(x)+v_{p}(y)$.
• p-adic values satisfy the archimedean inequality: $v_{p}(x+y)\leq \min\{v_{p}(x),v_{p}(y)\}$.
• Equality holds in the above so long as $v_{p}(x)\neq v_{p}(y)$.
• The fundamental theorem of arithmetic can be restated compactly using p-adic values: For all natural numbers n, $n=\prod _{p}p^{v_{p}(n)}$ where p ranges over all primes.
• p-adic values can be extended to the rational numbers by defining $v_{p}(x/y)=v_{p}(x)-v_{p}(y)$ for all integers x,y.
• Ostrowski's theorem states that the only absolute values on the field of rational numbers are the real absolute value (which some mathematicians view as the "prime at infinity") and the p-adic values.

p-adic values are used most commonly in number theory and algebra, especially in the theory of commutative rings.