21105731 has 4 divisors (see below), whose sum is σ = 21121464. Its totient is φ = 21090000.

The previous prime is 21105709. The next prime is 21105739. The reversal of 21105731 is 13750112.

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

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21105731 is a prime.

It is a super-2 number, since 2×21105731² = 890903762088722, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21105739) 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, 5645 + ... + 8606.

It is an arithmetic number, because the mean of its divisors is an integer number (5280366).

Almost surely, 2^21105731 is an apocalyptic number.

21105731 is a deficient number, since it is larger than the sum of its proper divisors (15733).

21105731 is a wasteful number, since it uses less digits than its factorization.

21105731 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 15732.

The product of its (nonzero) digits is 210, while the sum is 20.

The square root of 21105731 is about 4594.0974086321. The cubic root of 21105731 is about 276.3546646526.

Adding to 21105731 its reverse (13750112), we get a palindrome (34855843).

The spelling of 21105731 in words is "twenty-one million, one hundred five thousand, seven hundred thirty-one".