Test If Number Is Prime Number

Re: Mod Function

Googling through up a few formula.
This one appears to work.

=AND(MOD(A1,ROW(INDIRECT("2:"&INT(A1^0.5)))))

Re: Mod Function

Andy

Is that formula meant to test if a number is prime?

When I try it I get some strange results.

eg 2,3 - not prime, 15, 21,25, 33 - prime

Perhaps I didn't use the formula right, all I did was pop it in B1 and then enter various numbers in A1.

Re: Mod Function

Google found this too:
**********************************

POWER FORMULA TECHNIQUE

by Bob Umlas

This array formula returns TRUE if the number in cell A1 is a prime number.

=OR(A1=2,A1=3,ISNA(MATCH(TRUE,A1/ROW(INDIRECT("2:"&INT(SQRT(A1))))=
INT(A1/ROW(INDIRECT("2:"&INT(SQRT(A1))))),0)))

Use it as a conditional formatting formula, with A1 as the active cell
in the selection to be formatted.

Here's how Bob's amazing formula works. In a nutshell, the number is
divided by all potential prime factors, and the resulting array is tested
to see whether it contains a whole number. If is does, you have a prime
number. A limitation of this formula is that it cannot test numbers that
are greater than 65535^2. This is due to the array size constraint in
Excel 97/2000.

**********************************

Re: Mod Function

Sicarii

Again that's telling me 33 is prime.:confused:

Re: Test If Number Is Prime

I was in my prime at 33... :grin:

Re: Test If Number Is Prime

Oops I forget to add the cavet of Array formula.

I also note that numbers it deals with values greater than 5

Re: Test If Number Is Prime

Sorry, it's an array formula...

Re: Test If Number Is Prime

Ah, should of thought of that.:oops:

Was starting to think that the basic foundations of mathematics had seismically shifted overnight and my (ancient) maths degree was worthless.

Oh, wait a minute that degree is practically worthless.:roll:

Never got me a job anyway, and the most common comment is 'So you're a teacher then?'.:crying:

:dolphin: :dolphin: :dolphin: :dolphin:

Re: Test If Number Is Prime

That's a very clever and compact formula, Andy.