215561721 has 4 divisors (see below), whose sum is σ = 287415632. Its totient is φ = 143707812.

The previous prime is 215561701. The next prime is 215561747. The reversal of 215561721 is 127165512.

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 215561721 - 2¹⁵ = 215528953 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (215561701) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 35926951 + ... + 35926956.

It is an arithmetic number, because the mean of its divisors is an integer number (71853908).

Almost surely, 2^215561721 is an apocalyptic number.

It is an amenable number.

215561721 is a deficient number, since it is larger than the sum of its proper divisors (71853911).

215561721 is an equidigital number, since it uses as much as digits as its factorization.

215561721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 71853910.

The product of its digits is 4200, while the sum is 30.

The square root of 215561721 is about 14682.0203310035. The cubic root of 215561721 is about 599.5939113264.

The spelling of 215561721 in words is "two hundred fifteen million, five hundred sixty-one thousand, seven hundred twenty-one".