21305557 has 16 divisors (see below), whose sum is σ = 26353152. Its totient is φ = 16773120.

The previous prime is 21305551. The next prime is 21305563. The reversal of 21305557 is 75550312.

It is an interprime number because it is at equal distance from previous prime (21305551) and next prime (21305563).

It is a de Polignac number, because none of the positive numbers 2^k-21305557 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21305551) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 22932 + ... + 23842.

It is an arithmetic number, because the mean of its divisors is an integer number (1647072).

Almost surely, 2^21305557 is an apocalyptic number.

It is an amenable number.

21305557 is a deficient number, since it is larger than the sum of its proper divisors (5047595).

21305557 is a wasteful number, since it uses less digits than its factorization.

21305557 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1188.

The product of its (nonzero) digits is 5250, while the sum is 28.

The square root of 21305557 is about 4615.7942978430. The cubic root of 21305557 is about 277.2240884999.

Adding to 21305557 its reverse (75550312), we get a palindrome (96855869).

The spelling of 21305557 in words is "twenty-one million, three hundred five thousand, five hundred fifty-seven".