21423551 has 4 divisors (see below), whose sum is σ = 21439992. Its totient is φ = 21407112.

The previous prime is 21423539. The next prime is 21423559. The reversal of 21423551 is 15532412.

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 21423551 - 2⁶ = 21423487 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 (21423559) 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, 6080 + ... + 8933.

It is an arithmetic number, because the mean of its divisors is an integer number (5359998).

Almost surely, 2^21423551 is an apocalyptic number.

21423551 is a deficient number, since it is larger than the sum of its proper divisors (16441).

21423551 is a wasteful number, since it uses less digits than its factorization.

21423551 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 16440.

The product of its digits is 1200, while the sum is 23.

The square root of 21423551 is about 4628.5581988347. The cubic root of 21423551 is about 277.7349188927.

Adding to 21423551 its reverse (15532412), we get a palindrome (36955963).

The spelling of 21423551 in words is "twenty-one million, four hundred twenty-three thousand, five hundred fifty-one".