21121391 has 4 divisors (see below), whose sum is σ = 21358800. Its totient is φ = 20883984.

The previous prime is 21121381. The next prime is 21121403. The reversal of 21121391 is 19312112.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21121391 - 2¹⁰ = 21120367 is a prime.

It is a super-2 number, since 2×21121391² = 892226315549762, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21121381) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 118571 + ... + 118748.

It is an arithmetic number, because the mean of its divisors is an integer number (5339700).

Almost surely, 2^21121391 is an apocalyptic number.

21121391 is a deficient number, since it is larger than the sum of its proper divisors (237409).

21121391 is an equidigital number, since it uses as much as digits as its factorization.

21121391 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 237408.

The product of its digits is 108, while the sum is 20.

The square root of 21121391 is about 4595.8014535008. The cubic root of 21121391 is about 276.4229975008.

It can be divided in two parts, 21121 and 391, that added together give a palindrome (21512).

The spelling of 21121391 in words is "twenty-one million, one hundred twenty-one thousand, three hundred ninety-one".