21551020405 has 4 divisors (see below), whose sum is σ = 25861224492. Its totient is φ = 17240816320.

The previous prime is 21551020399. The next prime is 21551020409. The reversal of 21551020405 is 50402015512.

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 15832182276 + 5718838129 = 125826^2 + 75623^2 .

It is not a de Polignac number, because 21551020405 - 2³ = 21551020397 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21551020405.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21551020409) 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, 2155102036 + ... + 2155102045.

It is an arithmetic number, because the mean of its divisors is an integer number (6465306123).

Almost surely, 2^21551020405 is an apocalyptic number.

It is an amenable number.

21551020405 is a deficient number, since it is larger than the sum of its proper divisors (4310204087).

21551020405 is an equidigital number, since it uses as much as digits as its factorization.

21551020405 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4310204086.

The product of its (nonzero) digits is 2000, while the sum is 25.

Adding to 21551020405 its reverse (50402015512), we get a palindrome (71953035917).

The spelling of 21551020405 in words is "twenty-one billion, five hundred fifty-one million, twenty thousand, four hundred five".