It is possible to find out using mathematical methods whether a given integer is a prime number or not.
Yes, 25 009 991 is a prime number.
Indeed, the definition of a prime numbers is to have exactly two distinct positive divisors, 1 and itself. A number is a divisor of another number when the remainder of Euclid’s division of the second one by the first one is zero. Concerning the number 25 009 991, the only two divisors are 1 and 25 009 991. Therefore 25 009 991 is a prime number.
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Since 25009991 is a prime number, 25009991 is also a deficient number, that is to say 25009991 is a natural integer that is strictly larger than the sum of its proper divisors, i.e., the divisors of 25009991 without 25009991 itself (that is 1, by definition!).