21620605661 has 4 divisors (see below), whose sum is σ = 22892406012. Its totient is φ = 20348805312.

The previous prime is 21620605651. The next prime is 21620605711. The reversal of 21620605661 is 16650602612.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 8805007225 + 12815598436 = 93835^2 + 113206^2 .

It is a cyclic number.

It is not a de Polignac number, because 21620605661 - 2⁶ = 21620605597 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21620605631) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 635900150 + ... + 635900183.

It is an arithmetic number, because the mean of its divisors is an integer number (5723101503).

Almost surely, 2^21620605661 is an apocalyptic number.

It is an amenable number.

21620605661 is a deficient number, since it is larger than the sum of its proper divisors (1271800351).

21620605661 is a wasteful number, since it uses less digits than its factorization.

21620605661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1271800350.

The product of its (nonzero) digits is 25920, while the sum is 35.

The spelling of 21620605661 in words is "twenty-one billion, six hundred twenty million, six hundred five thousand, six hundred sixty-one".