21387251 has 2 divisors, whose sum is σ = 21387252. Its totient is φ = 21387250.

The previous prime is 21387227. The next prime is 21387257. The reversal of 21387251 is 15278312.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21387251 - 2¹⁸ = 21125107 is a prime.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21387257) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10693625 + 10693626.

It is an arithmetic number, because the mean of its divisors is an integer number (10693626).

Almost surely, 2^21387251 is an apocalyptic number.

21387251 is a deficient number, since it is larger than the sum of its proper divisors (1).

21387251 is an equidigital number, since it uses as much as digits as its factorization.

21387251 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 3360, while the sum is 29.

The square root of 21387251 is about 4624.6352288586. The cubic root of 21387251 is about 277.5779658103.

The spelling of 21387251 in words is "twenty-one million, three hundred eighty-seven thousand, two hundred fifty-one".