21557551 has 2 divisors, whose sum is σ = 21557552. Its totient is φ = 21557550.

The previous prime is 21557539. The next prime is 21557563. The reversal of 21557551 is 15575512.

It is a balanced prime because it is at equal distance from previous prime (21557539) and next prime (21557563).

It is a cyclic number.

It is not a de Polignac number, because 21557551 - 2⁵ = 21557519 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21557531) 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 as a sum of consecutive naturals, namely, 10778775 + 10778776.

It is an arithmetic number, because the mean of its divisors is an integer number (10778776).

Almost surely, 2^21557551 is an apocalyptic number.

21557551 is a deficient number, since it is larger than the sum of its proper divisors (1).

21557551 is an equidigital number, since it uses as much as digits as its factorization.

21557551 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 8750, while the sum is 31.

The square root of 21557551 is about 4643.0109842644. The cubic root of 21557551 is about 278.3127744465.

The spelling of 21557551 in words is "twenty-one million, five hundred fifty-seven thousand, five hundred fifty-one".