21779 has 4 divisors (see below), whose sum is σ = 22560. Its totient is φ = 21000.

The previous prime is 21773. The next prime is 21787. The reversal of 21779 is 97712.

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

It is a cyclic number.

It is not a de Polignac number, because 21779 - 2⁸ = 21523 is a prime.

It is a Duffinian number.

It is a plaindrome in base 6 and base 15.

It is not an unprimeable number, because it can be changed into a prime (21773) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 347 + ... + 404.

It is an arithmetic number, because the mean of its divisors is an integer number (5640).

2²¹⁷⁷⁹ is an apocalyptic number.

21779 is a gapful number since it is divisible by the number (29) formed by its first and last digit.

21779 is a deficient number, since it is larger than the sum of its proper divisors (781).

21779 is an equidigital number, since it uses as much as digits as its factorization.

21779 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 780.

The product of its digits is 882, while the sum is 26.

The square root of 21779 is about 147.5770984943. The cubic root of 21779 is about 27.9262515045.

The spelling of 21779 in words is "twenty-one thousand, seven hundred seventy-nine".