Number Theory

Prime checker

Is it prime? Exact for numbers up to 101810^{18}.

For checking your work after you’ve tried it yourself.

Step 1:
Type a number

Any whole number from 1 to 101810^{18}.

Step 2:
Press Check

Or press Enter.

Step 3:
Compare

See if it matches your answer, then open Do it by hand.
What it does.
Tells you whether a whole number is prime, meaning only 1 and itself divide it. If it isn't prime, it shows the smallest prime that divides it, so you can see why.
When it helps.
Checking a number you tested yourself, or making sure a number you're relying on really is prime.
What it won’t do.
It checks one number at a time and doesn't search for primes. It stops above 101810^{18}.
Example.
91 looks prime but isn't: 91=7×1391 = 7 \times 13.

Spaces and commas are ignored.

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