21780491 has 2 divisors, whose sum is σ = 21780492. Its totient is φ = 21780490.

The previous prime is 21780487. The next prime is 21780511. The reversal of 21780491 is 19408712.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21780491 - 2² = 21780487 is a prime.

It is not a weakly prime, because it can be changed into another prime (21780431) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10890245 + 10890246.

It is an arithmetic number, because the mean of its divisors is an integer number (10890246).

Almost surely, 2^21780491 is an apocalyptic number.

21780491 is a deficient number, since it is larger than the sum of its proper divisors (1).

21780491 is an equidigital number, since it uses as much as digits as its factorization.

21780491 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 4032, while the sum is 32.

The square root of 21780491 is about 4666.9573599938. The cubic root of 21780491 is about 279.2688877111.

It can be divided in two parts, 217 and 80491, that added together give a palindrome (80708).

The spelling of 21780491 in words is "twenty-one million, seven hundred eighty thousand, four hundred ninety-one".