21500021 has 2 divisors, whose sum is σ = 21500022. Its totient is φ = 21500020.

The previous prime is 21499981. The next prime is 21500023. The reversal of 21500021 is 12000512.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 13060996 + 8439025 = 3614^2 + 2905^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21500021 is a prime.

It is a super-3 number, since 3×21500021³ (a number of 23 digits) contains 333 as substring.

Together with 21500023, it forms a pair of twin primes.

It is a Chen prime.

It is a junction number, because it is equal to n+sod(n) for n = 21499975 and 21500011.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21500023) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10750010 + 10750011.

It is an arithmetic number, because the mean of its divisors is an integer number (10750011).

Almost surely, 2^21500021 is an apocalyptic number.

It is an amenable number.

21500021 is a deficient number, since it is larger than the sum of its proper divisors (1).

21500021 is an equidigital number, since it uses as much as digits as its factorization.

21500021 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 20, while the sum is 11.

The square root of 21500021 is about 4636.8115122355. The cubic root of 21500021 is about 278.0649788588.

Adding to 21500021 its reverse (12000512), we get a palindrome (33500533).

The spelling of 21500021 in words is "twenty-one million, five hundred thousand, twenty-one".