This add-in contains three worksheet functions.
Prime factorises a number in a single worksheet cell. If it can't find a factor, it states that the number
is a prime. The function works by trial division. I have included a ceiling in the function after which it stops. You
can increase the ceiling but the function will be slower.
NextPrime finds the next prime bigger than a given number.
FermatPseudo applies the Fermat test to find pseudoprimes.
The Fermat test is used to find out whether large numbers are prime.
| If base ^ (p – 1) mod p = 1 then p is probably a prime |
The test finds all prime numbers but also picks up a few others, which are actually composite.
These are called "pseudoprimes".
For small numbers, trial division is a much easier way to find prime numbers. For this reason I decided to eliminate
actual primes first and use the FermatPseudo function to find just pseudoprimes.
Excel is not well suited to factorise big numbers.
See beyond Excel.
If you want a simple table with the prime numbers highlighted, have a look at primes with xsl under my non-Excel pages.
Interesting things to do
Find the smallest pseudoprimes in base 10. Look for numbers that are pseudoprimes in several bases.
Prime.zip contains two files
| a) prime.xla – the add-in described above | |
| b) prime examples.xls – gives a few examples of the functions in the add-in. | |
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Download prime.zip (24kb) |
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The prime pages is an excellent site with a very good selection of links. |
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You might also like prime puzzles particularly number 12, which involves decimal repeat cycles. |
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