21021720165655 has 16 divisors (see below), whose sum is σ = 27640573363584. Its totient is φ = 15221173422880.

The previous prime is 21021720165533. The next prime is 21021720165661. The reversal of 21021720165655 is 55656102712012.

It is not a de Polignac number, because 21021720165655 - 2¹¹ = 21021720163607 is a prime.

It is a Duffinian number.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 841866577 + ... + 841891546.

It is an arithmetic number, because the mean of its divisors is an integer number (1727535835224).

Almost surely, 2^{21021720165655} is an apocalyptic number.

21021720165655 is a deficient number, since it is larger than the sum of its proper divisors (6618853197929).

21021720165655 is a wasteful number, since it uses less digits than its factorization.

21021720165655 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1683758366.

The product of its (nonzero) digits is 252000, while the sum is 43.

Adding to 21021720165655 its reverse (55656102712012), we get a palindrome (76677822877667).

The spelling of 21021720165655 in words is "twenty-one trillion, twenty-one billion, seven hundred twenty million, one hundred sixty-five thousand, six hundred fifty-five".