21497591821 has 2 divisors, whose sum is σ = 21497591822. Its totient is φ = 21497591820.

The previous prime is 21497591779. The next prime is 21497591833. The reversal of 21497591821 is 12819579412.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 18658194025 + 2839397796 = 136595^2 + 53286^2 .

It is a cyclic number.

It is not a de Polignac number, because 21497591821 - 2²³ = 21489203213 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21497591851) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10748795910 + 10748795911.

It is an arithmetic number, because the mean of its divisors is an integer number (10748795911).

Almost surely, 2^21497591821 is an apocalyptic number.

It is an amenable number.

21497591821 is a deficient number, since it is larger than the sum of its proper divisors (1).

21497591821 is an equidigital number, since it uses as much as digits as its factorization.

21497591821 is an evil number, because the sum of its binary digits is even.

The product of its digits is 362880, while the sum is 49.

The spelling of 21497591821 in words is "twenty-one billion, four hundred ninety-seven million, five hundred ninety-one thousand, eight hundred twenty-one".