21008457 has 8 divisors (see below), whose sum is σ = 31123680. Its totient is φ = 14005620.

The previous prime is 21008423. The next prime is 21008459. The reversal of 21008457 is 75480012.

It is not a de Polignac number, because 21008457 - 2⁷ = 21008329 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27), and also a Moran number because the ratio is a prime number: 778091 = 21008457 / (2 + 1 + 0 + 0 + 8 + 4 + 5 + 7).

It is a self number, because there is not a number n which added to its sum of digits gives 21008457.

It is not an unprimeable number, because it can be changed into a prime (21008459) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 389019 + ... + 389072.

It is an arithmetic number, because the mean of its divisors is an integer number (3890460).

Almost surely, 2^21008457 is an apocalyptic number.

21008457 is a gapful number since it is divisible by the number (27) formed by its first and last digit.

It is an amenable number.

21008457 is a deficient number, since it is larger than the sum of its proper divisors (10115223).

21008457 is an equidigital number, since it uses as much as digits as its factorization.

21008457 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 778100 (or 778094 counting only the distinct ones).

The product of its (nonzero) digits is 2240, while the sum is 27.

The square root of 21008457 is about 4583.4983364238. The cubic root of 21008457 is about 275.9294479403.

Adding to 21008457 its reverse (75480012), we get a palindrome (96488469).

The spelling of 21008457 in words is "twenty-one million, eight thousand, four hundred fifty-seven".