21011861 has 4 divisors (see below), whose sum is σ = 22628172. Its totient is φ = 19395552.

The previous prime is 21011843. The next prime is 21011869. The reversal of 21011861 is 16811012.

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 8444836 + 12567025 = 2906^2 + 3545^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21011861 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 (21011869) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 808136 + ... + 808161.

It is an arithmetic number, because the mean of its divisors is an integer number (5657043).

Almost surely, 2^21011861 is an apocalyptic number.

It is an amenable number.

21011861 is a deficient number, since it is larger than the sum of its proper divisors (1616311).

21011861 is a wasteful number, since it uses less digits than its factorization.

21011861 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1616310.

The product of its (nonzero) digits is 96, while the sum is 20.

The square root of 21011861 is about 4583.8696534697. The cubic root of 21011861 is about 275.9443500837.

Adding to 21011861 its reverse (16811012), we get a palindrome (37822873).

The spelling of 21011861 in words is "twenty-one million, eleven thousand, eight hundred sixty-one".