21123551 has 4 divisors (see below), whose sum is σ = 21135552. Its totient is φ = 21111552.

The previous prime is 21123533. The next prime is 21123589. The reversal of 21123551 is 15532112.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21123551 - 2¹⁴ = 21107167 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 (21123511) 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, 2786 + ... + 7071.

It is an arithmetic number, because the mean of its divisors is an integer number (5283888).

Almost surely, 2^21123551 is an apocalyptic number.

21123551 is a deficient number, since it is larger than the sum of its proper divisors (12001).

21123551 is an equidigital number, since it uses as much as digits as its factorization.

21123551 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 12000.

The product of its digits is 300, while the sum is 20.

The square root of 21123551 is about 4596.0364445901. The cubic root of 21123551 is about 276.4324200702.

Adding to 21123551 its reverse (15532112), we get a palindrome (36655663).

The spelling of 21123551 in words is "twenty-one million, one hundred twenty-three thousand, five hundred fifty-one".