Negative numbers N always have at least four factors: N*1=N and abs(N)*-1=N. Therefore, even if "natural number" weren't in the definition of a prime (which it is), no negative number would ever qualify. Therefore, the next-largest prime number to any negative number is always 2.
gmp_nextprime
(PHP 5 >= 5.2.0, PHP 7, PHP 8)
gmp_nextprime — Find next prime number
Return Values
Return the next prime number greater than num,
as a GMP number.
Examples
Example #1 gmp_nextprime() example
<?php
$prime1 = gmp_nextprime(10); // next prime number greater than 10
$prime2 = gmp_nextprime(-1000); // next prime number greater than -1000
echo gmp_strval($prime1) . "\n";
echo gmp_strval($prime2) . "\n";
?>The above example will output:
11 2
Notes
Note:
This function uses a probabilistic algorithm to identify primes and chances to get a composite number are extremely small.
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User Contributed Notes 3 notes
IceMetalPunk at gmail dot com ¶
7 years ago
Tommy Lynge Jrgensen ¶
8 years ago
Prime numbers are positive, non-zero numbers that have exactly two factors. No more, no less. So testing negative numbers doesn't really make any sense.
