21605247 has 12 divisors (see below), whose sum is σ = 31322304. Its totient is φ = 14350560.

The previous prime is 21605209. The next prime is 21605251. The reversal of 21605247 is 74250612.

It is not a de Polignac number, because 21605247 - 2⁶ = 21605183 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21605247.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21605267) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 1743 + ... + 6800.

It is an arithmetic number, because the mean of its divisors is an integer number (2610192).

Almost surely, 2^21605247 is an apocalyptic number.

21605247 is a deficient number, since it is larger than the sum of its proper divisors (9717057).

21605247 is a wasteful number, since it uses less digits than its factorization.

21605247 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 8830 (or 8827 counting only the distinct ones).

The product of its (nonzero) digits is 3360, while the sum is 27.

The square root of 21605247 is about 4648.1444684949. The cubic root of 21605247 is about 278.5178785768.

Adding to 21605247 its reverse (74250612), we get a palindrome (95855859).

The spelling of 21605247 in words is "twenty-one million, six hundred five thousand, two hundred forty-seven".