P-adic values
Given a prime number p, the p-adic value is the function, denoted <math>v_p</math>, 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): <math>v_p(x)=\max\{n:p^n\mid x\}</math>. For example, the p-adic values of 60 for p=2,3,5,7,11,13... are 2,1,1,0,0,0,.... One can associate with the p-adic valuation an absolute value <math>|n|_p=p^{-v_P(n)}</math>.
By convention, <math>v_p(0)=\infty</math> 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): <math>v_p(xy) = v_p(x) + v_p(y)</math>.
- p-adic values satisfy the archimedean inequality: <math>v_p(x+y) \le \min\{v_p(x),v_p(y)\}</math>.
- Equality holds in the above so long as <math>v_p(x)\ne v_p(y)</math>.
- The fundamental theorem of arithmetic can be restated compactly using p-adic values: For all natural numbers n, <math>n=\prod_pp^{v_p(n)}</math> where p ranges over all primes.
- p-adic values can be extended to the rational numbers by defining <math>v_p(x/y)=v_p(x)-v_p(y)</math> 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 absolute values described above.
p-adic values are used most commonly in number theory and algebra, especially in the theory of commutative rings.
Completing the field of rational numbers with respect to p-adic values yiels the field of p-adic numbers.