21730 has 16 divisors (see below), whose sum is σ = 40824. Its totient is φ = 8320.

The previous prime is 21727. The next prime is 21737. The reversal of 21730 is 3712.

It can be written as a sum of positive squares in 4 ways, for example, as 14161 + 7569 = 119^2 + 87^2 .

It is a plaindrome in base 15.

It is a self number, because there is not a number n which added to its sum of digits gives 21730.

It is not an unprimeable number, because it can be changed into a prime (21737) 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 7 ways as a sum of consecutive naturals, for example, 384 + ... + 436.

2²¹⁷³⁰ is an apocalyptic number.

21730 is a deficient number, since it is larger than the sum of its proper divisors (19094).

21730 is a wasteful number, since it uses less digits than its factorization.

21730 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 101.

The product of its (nonzero) digits is 42, while the sum is 13.

The square root of 21730 is about 147.4109900923. The cubic root of 21730 is about 27.9052922648.

The spelling of 21730 in words is "twenty-one thousand, seven hundred thirty".