21387729661 has 4 divisors (see below), whose sum is σ = 21389206300. Its totient is φ = 21386253024.

The previous prime is 21387729637. The next prime is 21387729667. The reversal of 21387729661 is 16692778312.

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 8115307225 + 13272422436 = 90085^2 + 115206^2 .

It is a cyclic number.

It is not a de Polignac number, because 21387729661 - 2⁷ = 21387729533 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21387729599 and 21387729608.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21387729667) 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, 716376 + ... + 745633.

It is an arithmetic number, because the mean of its divisors is an integer number (5347301575).

Almost surely, 2^21387729661 is an apocalyptic number.

It is an amenable number.

21387729661 is a deficient number, since it is larger than the sum of its proper divisors (1476639).

21387729661 is a wasteful number, since it uses less digits than its factorization.

21387729661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1476638.

The product of its digits is 1524096, while the sum is 52.

The spelling of 21387729661 in words is "twenty-one billion, three hundred eighty-seven million, seven hundred twenty-nine thousand, six hundred sixty-one".