21501561 has 4 divisors (see below), whose sum is σ = 28668752. Its totient is φ = 14334372.

The previous prime is 21501553. The next prime is 21501593. The reversal of 21501561 is 16510512.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is not a de Polignac number, because 21501561 - 2³ = 21501553 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21501511) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3583591 + ... + 3583596.

It is an arithmetic number, because the mean of its divisors is an integer number (7167188).

Almost surely, 2^21501561 is an apocalyptic number.

It is an amenable number.

21501561 is a deficient number, since it is larger than the sum of its proper divisors (7167191).

21501561 is an equidigital number, since it uses as much as digits as its factorization.

21501561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7167190.

The product of its (nonzero) digits is 300, while the sum is 21.

The square root of 21501561 is about 4636.9775716516. The cubic root of 21501561 is about 278.0716177646.

The spelling of 21501561 in words is "twenty-one million, five hundred one thousand, five hundred sixty-one".