21405591 has 6 divisors (see below), whose sum is σ = 30919200. Its totient is φ = 14270388.

The previous prime is 21405589. The next prime is 21405607. The reversal of 21405591 is 19550412.

It is not a de Polignac number, because 21405591 - 2¹ = 21405589 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21405511) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1189191 + ... + 1189208.

It is an arithmetic number, because the mean of its divisors is an integer number (5153200).

Almost surely, 2^21405591 is an apocalyptic number.

21405591 is a deficient number, since it is larger than the sum of its proper divisors (9513609).

21405591 is a wasteful number, since it uses less digits than its factorization.

21405591 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2378405 (or 2378402 counting only the distinct ones).

The product of its (nonzero) digits is 1800, while the sum is 27.

The square root of 21405591 is about 4626.6176630450. The cubic root of 21405591 is about 277.6572860470.

It can be divided in two parts, 2 and 1405591, that multiplied together give a palindrome (2811182).

The spelling of 21405591 in words is "twenty-one million, four hundred five thousand, five hundred ninety-one".