21208257 has 16 divisors (see below), whose sum is σ = 35908480. Its totient is φ = 12118896.

The previous prime is 21208183. The next prime is 21208259. The reversal of 21208257 is 75280212.

It is not a de Polignac number, because 21208257 - 2¹⁰ = 21207233 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21208257.

It is not an unprimeable number, because it can be changed into a prime (21208259) 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, 55918 + ... + 56295.

It is an arithmetic number, because the mean of its divisors is an integer number (2244280).

Almost surely, 2^21208257 is an apocalyptic number.

21208257 is a gapful number since it is divisible by the number (27) formed by its first and last digit.

It is an amenable number.

21208257 is a deficient number, since it is larger than the sum of its proper divisors (14700223).

21208257 is a wasteful number, since it uses less digits than its factorization.

21208257 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 112229 (or 112223 counting only the distinct ones).

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

The square root of 21208257 is about 4605.2423389003. The cubic root of 21208257 is about 276.8014277118.

Adding to 21208257 its reverse (75280212), we get a palindrome (96488469).

The spelling of 21208257 in words is "twenty-one million, two hundred eight thousand, two hundred fifty-seven".