20640391 has 2 divisors, whose sum is σ = 20640392. Its totient is φ = 20640390.

The previous prime is 20640331. The next prime is 20640401. The reversal of 20640391 is 19304602.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 20640391 - 2¹⁵ = 20607623 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (20640331) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10320195 + 10320196.

It is an arithmetic number, because the mean of its divisors is an integer number (10320196).

Almost surely, 2^20640391 is an apocalyptic number.

20640391 is a deficient number, since it is larger than the sum of its proper divisors (1).

20640391 is an equidigital number, since it uses as much as digits as its factorization.

20640391 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 1296, while the sum is 25.

The square root of 20640391 is about 4543.1697084745. The cubic root of 20640391 is about 274.3085261414.

Adding to 20640391 its reverse (19304602), we get a palindrome (39944993).

The spelling of 20640391 in words is "twenty million, six hundred forty thousand, three hundred ninety-one".