209660661 has 18 divisors (see below), whose sum is σ = 352287702. Its totient is φ = 119805840.

The previous prime is 209660641. The next prime is 209660681. The reversal of 209660661 is 166066902.

It is a happy number.

It is an interprime number because it is at equal distance from previous prime (209660641) and next prime (209660681).

It can be written as a sum of positive squares in only one way, i.e., 128119761 + 81540900 = 11319^2 + 9030^2 .

It is not a de Polignac number, because 209660661 - 2⁶ = 209660597 is a prime.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (209660641) by changing a digit.

It is a polite number, since it can be written in 17 ways as a sum of consecutive naturals, for example, 237270 + ... + 238151.

It is an arithmetic number, because the mean of its divisors is an integer number (19571539).

Almost surely, 2^209660661 is an apocalyptic number.

209660661 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

209660661 is a deficient number, since it is larger than the sum of its proper divisors (142627041).

209660661 is a wasteful number, since it uses less digits than its factorization.

209660661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 475441 (or 475431 counting only the distinct ones).

The product of its (nonzero) digits is 23328, while the sum is 36.

The square root of 209660661 is about 14479.6637046583. The cubic root of 209660661 is about 594.0718632233.

The spelling of 209660661 in words is "two hundred nine million, six hundred sixty thousand, six hundred sixty-one".