# On infinitude of primes of certain form. [duplicate]

We know that there are infinite number of primes so as there are infinite number of primes of the form $4n+3$ where $n\in Z^+$.

A note on Burton's book (Elementary Number Theory) somehow says that it is of high chance to expect that there are also infinite number of primes of the form $4n+1$. However it is not yet proven by the time the book was published.

A quick search on net gives infinitude of primes of the form $3n+1$ and $5n+1$.

The question is: The problem: Are there infinitely many number of primes of the form $4n+1$ still an open problem or it was already proven? If so, where can I find the proof? Thanks a lot.

## marked as duplicate by Travis, Mark Bennet, user133281, Claude Leibovici, Mark FantiniJan 14 '15 at 8:38

• Dirichlet's Theorem shows that there are infinitely many primes of the form $a n + b$ for coprime $a, b$: en.wikipedia.org/wiki/… – Travis Jan 14 '15 at 7:49
Hint: Suppose there are only finitely many such primes of the form $4n+1$, call them: $p_1, p_2,\cdots, p_n$. Its an exercise for you to "show" that: $p_{n+1} = 4(p_1p_2\cdots p_n)^2 + 1$ is a prime also of the form $4n+1$.