21820391 has 2 divisors, whose sum is σ = 21820392. Its totient is φ = 21820390.

The previous prime is 21820349. The next prime is 21820411. The reversal of 21820391 is 19302812.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21820391 - 2¹⁴ = 21804007 is a prime.

It is a super-2 number, since 2×21820391² = 952258926785762, which contains 22 as substring.

It is a self number, because there is not a number n which added to its sum of digits gives 21820391.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21820301) by changing a digit.

It is a good prime.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10910195 + 10910196.

It is an arithmetic number, because the mean of its divisors is an integer number (10910196).

Almost surely, 2^21820391 is an apocalyptic number.

21820391 is a deficient number, since it is larger than the sum of its proper divisors (1).

21820391 is an equidigital number, since it uses as much as digits as its factorization.

21820391 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 864, while the sum is 26.

The square root of 21820391 is about 4671.2301377689. The cubic root of 21820391 is about 279.4393159367.

The spelling of 21820391 in words is "twenty-one million, eight hundred twenty thousand, three hundred ninety-one".