The isPrime() Command

isprime.png

Command Summary

Returns true for prime arguments and false for composite arguments.

Command Syntax

isPrime(expression)

Menu Location

This command can't be found in any menu besides the command catalog.

Calculator Compatibility

This command works on all calculators.

The isPrime() command returns a Boolean value based on whether or not the passed argument is prime or not. True is returned for prime numbers and false is returned for composite numbers. A number is prime if its divisors are only 1 and itself. For instance, the number 23 is considered prime because no two positive integers but 1 and 23 can multiply to get 23. For the calculator, 1 isn't considered prime.

isPrime(23)
       true
isPrime(24)
       false

Algorithm

According to TI, the algorithm run by the calculators is as follows:

"The algorithms used by the TI-89 family, TI-92 family, and Voyage 200 calculators divides by successive primes through the largest one less than 216. It does not actually keep a table or use a sieve to create these divisors, but cyclically adds the sequence of increments 2, 2, 4, 2, 4, 2, 4, 6, 2, 6 to generate these primes plus a few extra harmless composites.

TI-92 Plus and TI-89 family start the same way, except that they stop this trial division after trial divisor 1021, then switch to a relatively fast Monte-Carlo test that determines whether the number is certainly composite or is almost certainly prime. The isPrime() function stops at this point returning either false or (almost certainly) true."

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