21991861 has 4 divisors (see below), whose sum is σ = 22071532. Its totient is φ = 21912192.

The previous prime is 21991847. The next prime is 21991883. The reversal of 21991861 is 16819912.

It is the 1915-th star number.

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 8814961 + 13176900 = 2969^2 + 3630^2 .

It is a cyclic number.

It is not a de Polignac number, because 21991861 - 2⁷ = 21991733 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 21991861.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21991891) 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, 39420 + ... + 39973.

It is an arithmetic number, because the mean of its divisors is an integer number (5517883).

Almost surely, 2^21991861 is an apocalyptic number.

It is an amenable number.

21991861 is a deficient number, since it is larger than the sum of its proper divisors (79671).

21991861 is an equidigital number, since it uses as much as digits as its factorization.

21991861 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 79670.

The product of its digits is 7776, while the sum is 37.

The square root of 21991861 is about 4689.5480592484. The cubic root of 21991861 is about 280.1693745639.

The spelling of 21991861 in words is "twenty-one million, nine hundred ninety-one thousand, eight hundred sixty-one".