21509723 has 2 divisors, whose sum is σ = 21509724. Its totient is φ = 21509722.

The previous prime is 21509717. The next prime is 21509731. The reversal of 21509723 is 32790512.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21509723 - 2⁶ = 21509659 is a prime.

It is not a weakly prime, because it can be changed into another prime (21509713) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10754861 + 10754862.

It is an arithmetic number, because the mean of its divisors is an integer number (10754862).

Almost surely, 2^21509723 is an apocalyptic number.

21509723 is a deficient number, since it is larger than the sum of its proper divisors (1).

21509723 is an equidigital number, since it uses as much as digits as its factorization.

21509723 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 3780, while the sum is 29.

The square root of 21509723 is about 4637.8575872918. The cubic root of 21509723 is about 278.1067986742.

The spelling of 21509723 in words is "twenty-one million, five hundred nine thousand, seven hundred twenty-three".