21751021 has 2 divisors, whose sum is σ = 21751022. Its totient is φ = 21751020.

The previous prime is 21751019. The next prime is 21751027. The reversal of 21751021 is 12015712.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 18826921 + 2924100 = 4339^2 + 1710^2 .

It is a cyclic number.

It is not a de Polignac number, because 21751021 - 2¹ = 21751019 is a prime.

Together with 21751019, it forms a pair of twin primes.

It is a self number, because there is not a number n which added to its sum of digits gives 21751021.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21751027) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10875510 + 10875511.

It is an arithmetic number, because the mean of its divisors is an integer number (10875511).

Almost surely, 2^21751021 is an apocalyptic number.

It is an amenable number.

21751021 is a deficient number, since it is larger than the sum of its proper divisors (1).

21751021 is an equidigital number, since it uses as much as digits as its factorization.

21751021 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 140, while the sum is 19.

The square root of 21751021 is about 4663.7989879496. The cubic root of 21751021 is about 279.1428763371.

Adding to 21751021 its reverse (12015712), we get a palindrome (33766733).

The spelling of 21751021 in words is "twenty-one million, seven hundred fifty-one thousand, twenty-one".