21508321 has 2 divisors, whose sum is σ = 21508322. Its totient is φ = 21508320.

The previous prime is 21508309. The next prime is 21508337. The reversal of 21508321 is 12380512.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 20802721 + 705600 = 4561^2 + 840^2 .

It is a cyclic number.

It is not a de Polignac number, because 21508321 - 2⁹ = 21507809 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21508292 and 21508301.

It is not a weakly prime, because it can be changed into another prime (21508121) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10754160 + 10754161.

It is an arithmetic number, because the mean of its divisors is an integer number (10754161).

Almost surely, 2^21508321 is an apocalyptic number.

It is an amenable number.

21508321 is a deficient number, since it is larger than the sum of its proper divisors (1).

21508321 is an equidigital number, since it uses as much as digits as its factorization.

21508321 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 480, while the sum is 22.

The square root of 21508321 is about 4637.7064374538. The cubic root of 21508321 is about 278.1007562254.

Adding to 21508321 its reverse (12380512), we get a palindrome (33888833).

The spelling of 21508321 in words is "twenty-one million, five hundred eight thousand, three hundred twenty-one".