21320551 has 4 divisors (see below), whose sum is σ = 24366352. Its totient is φ = 18274752.

The previous prime is 21320543. The next prime is 21320557. The reversal of 21320551 is 15502312.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21320551 - 2³ = 21320543 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21320557) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1522890 + ... + 1522903.

It is an arithmetic number, because the mean of its divisors is an integer number (6091588).

Almost surely, 2^21320551 is an apocalyptic number.

21320551 is a deficient number, since it is larger than the sum of its proper divisors (3045801).

21320551 is an equidigital number, since it uses as much as digits as its factorization.

21320551 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3045800.

The product of its (nonzero) digits is 300, while the sum is 19.

The square root of 21320551 is about 4617.4182180088. The cubic root of 21320551 is about 277.2891063301.

Adding to 21320551 its reverse (15502312), we get a palindrome (36822863).

The spelling of 21320551 in words is "twenty-one million, three hundred twenty thousand, five hundred fifty-one".