215661 has 4 divisors (see below), whose sum is σ = 287552. Its totient is φ = 143772.

The previous prime is 215659. The next prime is 215681. The reversal of 215661 is 166512.

It is a happy number.

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 215661 - 2¹ = 215659 is a prime.

It is a D-number.

It is a Duffinian number.

215661 is a lucky number.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 215661.

It is not an unprimeable number, because it can be changed into a prime (215681) 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, 35941 + ... + 35946.

It is an arithmetic number, because the mean of its divisors is an integer number (71888).

2²¹⁵⁶⁶¹ is an apocalyptic number.

It is an amenable number.

215661 is a deficient number, since it is larger than the sum of its proper divisors (71891).

215661 is an equidigital number, since it uses as much as digits as its factorization.

215661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 71890.

The product of its digits is 360, while the sum is 21.

The square root of 215661 is about 464.3931524043. The cubic root of 215661 is about 59.9685946757.

The spelling of 215661 in words is "two hundred fifteen thousand, six hundred sixty-one".