661209 has 4 divisors (see below), whose sum is σ = 881616. Its totient is φ = 440804.

The previous prime is 661201. The next prime is 661217. The reversal of 661209 is 902166.

661209 = T₂₆₂ + T₂₆₃ + ... + T₂₇₉.

661209 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

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 (661201) and next prime (661217).

It is a cyclic number.

It is not a de Polignac number, because 661209 - 2³ = 661201 is a prime.

It is a D-number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 661209.

It is not an unprimeable number, because it can be changed into a prime (661201) 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, 110199 + ... + 110204.

It is an arithmetic number, because the mean of its divisors is an integer number (220404).

2⁶⁶¹²⁰⁹ is an apocalyptic number.

It is an amenable number.

661209 is a deficient number, since it is larger than the sum of its proper divisors (220407).

661209 is a wasteful number, since it uses less digits than its factorization.

661209 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 220406.

The product of its (nonzero) digits is 648, while the sum is 24.

The square root of 661209 is about 813.1475880798. The cubic root of 661209 is about 87.1190074352.

It can be divided in two parts, 66 and 1209, that added together give a triangular number (1275 = T₅₀).

The spelling of 661209 in words is "six hundred sixty-one thousand, two hundred nine".