The largest Prime number under 10,000,000 is 9,999,991
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The largest prime under 20 is 19.
Prime numbers can't have whole number square roots, of course, but the largest prime number under 400 is 397.
997
because it has no other denominators beside 1 and itself
A prime number can be less than 20, but it is not a requirement that the number has to be under 20 to be considered prime. Here is a list of all the prime numbers under 20: 2, 3, 5, 7, 11, 13, 17, and 19 .