21021551 has 4 divisors (see below), whose sum is σ = 21036504. Its totient is φ = 21006600.

The previous prime is 21021529. The next prime is 21021569. The reversal of 21021551 is 15512012.

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

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21021551 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 (21021251) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 5120 + ... + 8261.

It is an arithmetic number, because the mean of its divisors is an integer number (5259126).

Almost surely, 2^21021551 is an apocalyptic number.

21021551 is a deficient number, since it is larger than the sum of its proper divisors (14953).

21021551 is a wasteful number, since it uses less digits than its factorization.

21021551 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 14952.

The product of its (nonzero) digits is 100, while the sum is 17.

The square root of 21021551 is about 4584.9264988656. The cubic root of 21021551 is about 275.9867624751.

Adding to 21021551 its reverse (15512012), we get a palindrome (36533563).

The spelling of 21021551 in words is "twenty-one million, twenty-one thousand, five hundred fifty-one".