21391651 has 2 divisors, whose sum is σ = 21391652. Its totient is φ = 21391650.

The previous prime is 21391649. The next prime is 21391703. The reversal of 21391651 is 15619312.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21391651 - 2¹ = 21391649 is a prime.

Together with 21391649, it forms a pair of twin primes.

It is a Chen prime.

It is equal to p_1353169 and since 21391651 and 1353169 have the same sum of digits, it is a Honaker prime.

It is not a weakly prime, because it can be changed into another prime (21391621) 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 as a sum of consecutive naturals, namely, 10695825 + 10695826.

It is an arithmetic number, because the mean of its divisors is an integer number (10695826).

Almost surely, 2^21391651 is an apocalyptic number.

21391651 is a deficient number, since it is larger than the sum of its proper divisors (1).

21391651 is an equidigital number, since it uses as much as digits as its factorization.

21391651 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 1620, while the sum is 28.

The square root of 21391651 is about 4625.1109175889. The cubic root of 21391651 is about 277.5969998804.

The spelling of 21391651 in words is "twenty-one million, three hundred ninety-one thousand, six hundred fifty-one".