Mathematics : if one divisor of a dividend is greater than the square root of the dividend, then the quotient is smaller than the square root of the dividend.

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We have ij = n,
i > 0, j > 0

if i > sqrt(n)
ij > sqrt(n) j
n > sqrt(n) j
sqrt(n) > j

Hence we have to check the primality of a number by trying to divide it only upto the square root of the number.

Thank you,
Praveen Kumar Sirohiwal.

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