The problem with your algorythm is that if you square a prime, you get a number divisible only by 1, the prime and the squared number itself. (Ex. 11 squared is 121) This number will not be found by your application, but is NOT a prime. What you need to add is a line to check whether or not the square root of the number is equal to the integer of the square root. If not, then, together with your process, you'll be fine.
Alan wrote:
Prime Numbers
14-Sep-08
Hi
I've been using this formula to identify whether a number is a prime number
or not. It works, but it's a bit long and incongruous
=IF(A1<8,"2, 3, 5, and 7 are prime numbers. Please enter a number greater
than
8",IF(OR(MOD($A$1,2)=0,MOD($A$1,3)=0,MOD($A$1,4)=0,MOD(A1,5)=0,MOD(A1,6)=0,MOD(A1,7)=0,MOD(A1,8)=0,MOD(A1,9)=0),"Not
Prime","Prime")
Can anyone suggest a better way
Thanks in advance
Alan.
Previous Posts In This Thread:
Prime Numbers
Hi
I've been using this formula to identify whether a number is a prime number
or not. It works, but it's a bit long and incongruous
=IF(A1<8,"2, 3, 5, and 7 are prime numbers. Please enter a number greater
than
8",IF(OR(MOD($A$1,2)=0,MOD($A$1,3)=0,MOD($A$1,4)=0,MOD(A1,5)=0,MOD(A1,6)=0,MOD(A1,7)=0,MOD(A1,8)=0,MOD(A1,9)=0),"Not
Prime","Prime")
Can anyone suggest a better way
Thanks in advance
Alan.
Try a formula like the following array
Try a formula like the following array formula
=IF(AND((MOD($A$1,ROW(INDIRECT("2:"&$A$1-1)))<>0)),"prime","not prime"
This will return "prime" if A1 is prime or "not prime" if it is not. Note
that this is a array formula, so you must press CTRL SHIFT ENTER rather than
just ENTER when you first enter the formula and whenever you edit it later.
If you do this properly, Excel will display the formula enclosed in curly
braces {}. The formula will not work if you do not enter it with CTRL SHIFT
ENTER. See
http://www.cpearson.com/Excel/arrayformulas.aspx for much more
info about array formulas
--
Cordially
Chip Pearso
Microsoft Most Valuable Professiona
Excel Product Grou
Pearson Software Consulting, LL
www.cpearson.co
(email on web site
Just for fun, the following array formula will test for prime twins, returning
Just for fun, the following array formula will test for prime twins,
returning "prime", "prime twin", or "not prime"
=IF(AND((MOD($A$1,ROW(INDIRECT("2:"&$A$1-1)))<>0)),IF(OR(AND((MOD($A$1-2,ROW(INDIRECT("2:"&$A$1-3)))<>0)),AND((MOD($A$1+2,ROW(INDIRECT("2:"&$A$1+1)))<>0))),"prime
twin","prime"),"not prime"
Enter with CTRL SHIFT ENTER
--
Cordially
Chip Pearso
Microsoft Most Valuable Professiona
Excel Product Grou
Pearson Software Consulting, LL
www.cpearson.co
(email on web site
Re: Prime Numbers
... and in case anyone else is interested, a 'prime twin' or 'twin prime' is
2 prime numbers that differ by 2 (e.g. 5 and 7; 11 and 13) - the smallest
number they can differ by (one exception ... 2 and 3).
Thanks ... a fascinating formula ... and I learned something today.
Re: Prime Numbers
Perhaps have a look a
http://www.dailydoseofexcel.com/archives/2005/06/30/is-this-number-prime
-
met vriendelijke groetje
(e-mail address removed)
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