21680251 has 2 divisors, whose sum is σ = 21680252. Its totient is φ = 21680250.

The previous prime is 21680249. The next prime is 21680257. The reversal of 21680251 is 15208612.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21680251 - 2¹ = 21680249 is a prime.

Together with 21680249, it forms a pair of twin primes.

It is not a weakly prime, because it can be changed into another prime (21680257) 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, 10840125 + 10840126.

It is an arithmetic number, because the mean of its divisors is an integer number (10840126).

Almost surely, 2^21680251 is an apocalyptic number.

21680251 is a deficient number, since it is larger than the sum of its proper divisors (1).

21680251 is an equidigital number, since it uses as much as digits as its factorization.

21680251 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 960, while the sum is 25.

The square root of 21680251 is about 4656.2056440840. The cubic root of 21680251 is about 278.8398038924.

Adding to 21680251 its reverse (15208612), we get a palindrome (36888863).

The spelling of 21680251 in words is "twenty-one million, six hundred eighty thousand, two hundred fifty-one".