21643505 has 16 divisors (see below), whose sum is σ = 28071120. Its totient is φ = 15925248.

The previous prime is 21643429. The next prime is 21643507. The reversal of 21643505 is 50534612.

It can be written as a sum of positive squares in 8 ways, for example, as 5779216 + 15864289 = 2404^2 + 3983^2 .

It is a cyclic number.

It is not a de Polignac number, because 21643505 - 2¹⁸ = 21381361 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21643507) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 27761 + ... + 28529.

It is an arithmetic number, because the mean of its divisors is an integer number (1754445).

Almost surely, 2^21643505 is an apocalyptic number.

It is an amenable number.

21643505 is a deficient number, since it is larger than the sum of its proper divisors (6427615).

21643505 is a wasteful number, since it uses less digits than its factorization.

21643505 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1220.

The product of its (nonzero) digits is 3600, while the sum is 26.

The square root of 21643505 is about 4652.2580538917. The cubic root of 21643505 is about 278.6821789998.

The spelling of 21643505 in words is "twenty-one million, six hundred forty-three thousand, five hundred five".