21729780 has 36 divisors (see below), whose sum is σ = 65914212. Its totient is φ = 5794560.

The previous prime is 21729751. The next prime is 21729781. The reversal of 21729780 is 8792712.

It can be written as a sum of positive squares in 2 ways, for example, as 2702736 + 19027044 = 1644^2 + 4362^2 .

It is a tau number, because it is divible by the number of its divisors (36).

It is a Harshad number since it is a multiple of its sum of digits (36).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21729781) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 60181 + ... + 60540.

Almost surely, 2^21729780 is an apocalyptic number.

21729780 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21729780 is an abundant number, since it is smaller than the sum of its proper divisors (44184432).

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

21729780 is a wasteful number, since it uses less digits than its factorization.

21729780 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 120736 (or 120731 counting only the distinct ones).

The product of its (nonzero) digits is 14112, while the sum is 36.

The square root of 21729780 is about 4661.5212109353. The cubic root of 21729780 is about 279.0519809110.

The spelling of 21729780 in words is "twenty-one million, seven hundred twenty-nine thousand, seven hundred eighty".