205721661 has 4 divisors (see below), whose sum is σ = 274295552. Its totient is φ = 137147772.

The previous prime is 205721651. The next prime is 205721671. The reversal of 205721661 is 166127502.

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 an interprime number because it is at equal distance from previous prime (205721651) and next prime (205721671).

It is not a de Polignac number, because 205721661 - 2⁷ = 205721533 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (205721651) 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, 34286941 + ... + 34286946.

It is an arithmetic number, because the mean of its divisors is an integer number (68573888).

Almost surely, 2^205721661 is an apocalyptic number.

It is an amenable number.

205721661 is a deficient number, since it is larger than the sum of its proper divisors (68573891).

205721661 is an equidigital number, since it uses as much as digits as its factorization.

205721661 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 68573890.

The product of its (nonzero) digits is 5040, while the sum is 30.

The square root of 205721661 is about 14343.0004183225. Note that the first 3 decimals coincide. The cubic root of 205721661 is about 590.3279427440.

The spelling of 205721661 in words is "two hundred five million, seven hundred twenty-one thousand, six hundred sixty-one".