21780861 has 4 divisors (see below), whose sum is σ = 29041152. Its totient is φ = 14520572.

The previous prime is 21780823. The next prime is 21780887. The reversal of 21780861 is 16808712.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21780861 - 2¹⁰ = 21779837 is a prime.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21780361) 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, 3630141 + ... + 3630146.

It is an arithmetic number, because the mean of its divisors is an integer number (7260288).

Almost surely, 2^21780861 is an apocalyptic number.

It is an amenable number.

21780861 is a deficient number, since it is larger than the sum of its proper divisors (7260291).

21780861 is an equidigital number, since it uses as much as digits as its factorization.

21780861 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7260290.

The product of its (nonzero) digits is 5376, while the sum is 33.

The square root of 21780861 is about 4666.9970002133. The cubic root of 21780861 is about 279.2704690789.

The spelling of 21780861 in words is "twenty-one million, seven hundred eighty thousand, eight hundred sixty-one".