21309623 has 2 divisors, whose sum is σ = 21309624. Its totient is φ = 21309622.

The previous prime is 21309611. The next prime is 21309643. The reversal of 21309623 is 32690312.

It is a weak prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21309623 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21309592 and 21309601.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21309643) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10654811 + 10654812.

It is an arithmetic number, because the mean of its divisors is an integer number (10654812).

Almost surely, 2^21309623 is an apocalyptic number.

21309623 is a deficient number, since it is larger than the sum of its proper divisors (1).

21309623 is an equidigital number, since it uses as much as digits as its factorization.

21309623 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 1944, while the sum is 26.

The square root of 21309623 is about 4616.2347210687. The cubic root of 21309623 is about 277.2417227326.

Adding to 21309623 its reverse (32690312), we get a palindrome (53999935).

The spelling of 21309623 in words is "twenty-one million, three hundred nine thousand, six hundred twenty-three".