21720145 has 4 divisors (see below), whose sum is σ = 26064180. Its totient is φ = 17376112.

The previous prime is 21720137. The next prime is 21720197. The reversal of 21720145 is 54102712.

It is a happy number.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 2430481 + 19289664 = 1559^2 + 4392^2 .

It is a cyclic number.

It is not a de Polignac number, because 21720145 - 2³ = 21720137 is a prime.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2172010 + ... + 2172019.

It is an arithmetic number, because the mean of its divisors is an integer number (6516045).

Almost surely, 2^21720145 is an apocalyptic number.

It is an amenable number.

21720145 is a deficient number, since it is larger than the sum of its proper divisors (4344035).

21720145 is an equidigital number, since it uses as much as digits as its factorization.

21720145 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4344034.

The product of its (nonzero) digits is 560, while the sum is 22.

The square root of 21720145 is about 4660.4876354304. The cubic root of 21720145 is about 279.0107308643.

Adding to 21720145 its reverse (54102712), we get a palindrome (75822857).

The spelling of 21720145 in words is "twenty-one million, seven hundred twenty thousand, one hundred forty-five".