Talk:Prime number
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This is an old revision of this page, as edited by Isaac4given (Talk | contribs) at 10:28, 10 January 2008. It may differ significantly from current revision.
Primality testing - Problems with the number 4
When testing whether a number N is divisible by s, using values of s from 2 to N - 1, when N is 4, the method correctly determines that 4 is not prime, because it is divisible by 2. This example is in the C programming language.
for (s = 2; s < N - 1; ++s) {
if (N % s == 0) {
printf("N is not a prime number.");
}
}
However, when using values of m from 2 to the square root of n, when n is 4, an error can occur. Since 2 is the square root of 4, the test range is 2 to 2, and, if the range is exclusive, as in a for-loop, the whole test is skipped, and 4 is incorrectly determined to be a prime number.
for (s = 2; s < sqrt(N); ++s) {
if (N % s == 0) {
printf("N is not a prime number.");
}
}
Isaac4given 11:28, 10 January 2008 (EST)
