21756 has 36 divisors (see below), whose sum is σ = 60648. Its totient is φ = 6048.

The previous prime is 21751. The next prime is 21757. The reversal of 21756 is 65712.

It is a Harshad number since it is a multiple of its sum of digits (21).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21751) by changing a digit.

21756 is an untouchable number, because it is not equal to the sum of proper divisors of any number.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 570 + ... + 606.

2²¹⁷⁵⁶ is an apocalyptic number.

It is a pronic number, being equal to 147×148.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21756, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (30324).

21756 is an abundant number, since it is smaller than the sum of its proper divisors (38892).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21756 is a wasteful number, since it uses less digits than its factorization.

21756 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 58 (or 49 counting only the distinct ones).

The product of its digits is 420, while the sum is 21.

The square root of 21756 is about 147.4991525399. The cubic root of 21756 is about 27.9164174132.

Subtracting 21756 from its reverse (65712), we obtain a triangular number (43956 = T₂₉₆).

It can be divided in two parts, 21 and 756, that multiplied together give a square (15876 = 126²).

The spelling of 21756 in words is "twenty-one thousand, seven hundred fifty-six".