Trying to do a non-rigorous direct proof

1. Aug 10, 2013

ull

Statements:
x is an integer
x is a prime number if x doesnt consist of any prime factors ≤√x

Proof:
Since (√x + 1) * (√x + 1) > √x * √x
x must be a prime

Questions:

Whould you consider this a non-rigorous direct proof?
If not, what does it lack?
Is this a good approach trying to prove it?

The proof was meant to be like this:

Since √(x + 1) * √(x + 1) > √x * √x
x must be a prime

Last edited: Aug 10, 2013
2. Aug 10, 2013

Number Nine

Recall that every integer >1 has a (unique) prime factorization. Now, suppose that $x$ is not prime, and further has no prime factor $p_1 \leq \sqrt{x}$ (so that $p_1 > \sqrt{x}$); then $x$ has factorization $x = p_1???$. What possible values can $?$ take? (Hint: what happens if $? > \sqrt{x}$)

3. Aug 10, 2013

ull

Please dont post any more clues just jet im trying to figure it out it :):)
I dont know if this translates very well, but: "I´m going to sleep on it " ;)

Last edited: Aug 10, 2013
4. Aug 10, 2013

verty

I know you said no more hints, but this hint is just too useful not to suggest.

Hint: write the smallest divisor (not counting 1 of course) as $\sqrt{x} + ε$.

Hmm, I wonder if this counts as a direct proof, probably not.

5. Aug 11, 2013

jbriggs444

It is always a good idea to start with a proper statement of what it is you are trying to prove.

"For all positive integer x, if x has no prime factors less than or equal to its square root then x is prime"

One problem with this formulation is that it is false. The positive integer 1 is a counter-example.