21720243 has 8 divisors (see below), whose sum is σ = 29894656. Its totient is φ = 14013000.

The previous prime is 21720241. The next prime is 21720247. The reversal of 21720243 is 34202712.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21720243 - 2¹ = 21720241 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21720241) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 116683 + ... + 116868.

It is an arithmetic number, because the mean of its divisors is an integer number (3736832).

Almost surely, 2^21720243 is an apocalyptic number.

21720243 is a deficient number, since it is larger than the sum of its proper divisors (8174413).

21720243 is a wasteful number, since it uses less digits than its factorization.

21720243 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 233585.

The product of its (nonzero) digits is 672, while the sum is 21.

The square root of 21720243 is about 4660.4981493398. The cubic root of 21720243 is about 279.0111504902.

Adding to 21720243 its reverse (34202712), we get a palindrome (55922955).

The spelling of 21720243 in words is "twenty-one million, seven hundred twenty thousand, two hundred forty-three".