21021391 has 6 divisors (see below), whose sum is σ = 22186392. Its totient is φ = 19914660.

The previous prime is 21021389. The next prime is 21021419. The reversal of 21021391 is 19312012.

It is not a de Polignac number, because 21021391 - 2¹ = 21021389 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (19).

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21021331) 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 5 ways as a sum of consecutive naturals, for example, 28755 + ... + 29476.

It is an arithmetic number, because the mean of its divisors is an integer number (3697732).

Almost surely, 2^21021391 is an apocalyptic number.

21021391 is a deficient number, since it is larger than the sum of its proper divisors (1165001).

21021391 is an equidigital number, since it uses as much as digits as its factorization.

21021391 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 58269 (or 58250 counting only the distinct ones).

The product of its (nonzero) digits is 108, while the sum is 19.

The square root of 21021391 is about 4584.9090503520. The cubic root of 21021391 is about 275.9860622731.

It can be divided in two parts, 21021 and 391, that added together give a palindrome (21412).

The spelling of 21021391 in words is "twenty-one million, twenty-one thousand, three hundred ninety-one".