21775 has 12 divisors (see below), whose sum is σ = 29512. Its totient is φ = 15840.

The previous prime is 21773. The next prime is 21787. The reversal of 21775 is 57712.

It is not a de Polignac number, because 21775 - 2¹ = 21773 is a prime.

It is a hoax number, since the sum of its digits (22) coincides with the sum of the digits of its distinct prime factors.

It is an Ulam number.

It is a Duffinian number.

It is a zygodrome in base 4.

It is a congruent number.

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

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 292 + ... + 358.

2²¹⁷⁷⁵ is an apocalyptic number.

21775 is a gapful number since it is divisible by the number (25) formed by its first and last digit.

21775 is a deficient number, since it is larger than the sum of its proper divisors (7737).

21775 is a wasteful number, since it uses less digits than its factorization.

21775 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 90 (or 85 counting only the distinct ones).

The product of its digits is 490, while the sum is 22.

The square root of 21775 is about 147.5635456337. The cubic root of 21775 is about 27.9245417252.

Subtracting 21775 from its reverse (57712), we obtain a cube (35937 = 33³).

It can be divided in two parts, 217 and 75, that added together give a palindrome (292).

The spelling of 21775 in words is "twenty-one thousand, seven hundred seventy-five".