21605617721 has 2 divisors, whose sum is σ = 21605617722. Its totient is φ = 21605617720.

The previous prime is 21605617703. The next prime is 21605617739. The reversal of 21605617721 is 12771650612.

It is a balanced prime because it is at equal distance from previous prime (21605617703) and next prime (21605617739).

It can be written as a sum of positive squares in only one way, i.e., 15967860496 + 5637757225 = 126364^2 + 75085^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21605617721 is a prime.

It is not a weakly prime, because it can be changed into another prime (21605617751) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10802808860 + 10802808861.

It is an arithmetic number, because the mean of its divisors is an integer number (10802808861).

Almost surely, 2^21605617721 is an apocalyptic number.

It is an amenable number.

21605617721 is a deficient number, since it is larger than the sum of its proper divisors (1).

21605617721 is an equidigital number, since it uses as much as digits as its factorization.

21605617721 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 35280, while the sum is 38.

The spelling of 21605617721 in words is "twenty-one billion, six hundred five million, six hundred seventeen thousand, seven hundred twenty-one".