21751991 has 2 divisors, whose sum is σ = 21751992. Its totient is φ = 21751990.

The previous prime is 21751981. The next prime is 21751999. The reversal of 21751991 is 19915712.

It is a strong prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21751991 is a prime.

It is a super-2 number, since 2×21751991² = 946298224928162, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21751999) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10875995 + 10875996.

It is an arithmetic number, because the mean of its divisors is an integer number (10875996).

Almost surely, 2^21751991 is an apocalyptic number.

21751991 is a deficient number, since it is larger than the sum of its proper divisors (1).

21751991 is an equidigital number, since it uses as much as digits as its factorization.

21751991 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 5670, while the sum is 35.

The square root of 21751991 is about 4663.9029792653. The cubic root of 21751991 is about 279.1470257908.

The spelling of 21751991 in words is "twenty-one million, seven hundred fifty-one thousand, nine hundred ninety-one".