21641561 has 4 divisors (see below), whose sum is σ = 22914612. Its totient is φ = 20368512.

The previous prime is 21641537. The next prime is 21641579. The reversal of 21641561 is 16514612.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 17799961 + 3841600 = 4219^2 + 1960^2 .

It is a cyclic number.

It is not a de Polignac number, because 21641561 - 2¹⁰ = 21640537 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21641561.

It is not an unprimeable number, because it can be changed into a prime (21641591) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 636500 + ... + 636533.

It is an arithmetic number, because the mean of its divisors is an integer number (5728653).

Almost surely, 2^21641561 is an apocalyptic number.

It is an amenable number.

21641561 is a deficient number, since it is larger than the sum of its proper divisors (1273051).

21641561 is a wasteful number, since it uses less digits than its factorization.

21641561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1273050.

The product of its digits is 1440, while the sum is 26.

The square root of 21641561 is about 4652.0491183993. The cubic root of 21641561 is about 278.6738350898.

The spelling of 21641561 in words is "twenty-one million, six hundred forty-one thousand, five hundred sixty-one".