661720 has 32 divisors (see below), whose sum is σ = 1516320. Its totient is φ = 259840.

The previous prime is 661697. The next prime is 661721. The reversal of 661720 is 27166.

It is an Ulam number.

It is a junction number, because it is equal to n+sod(n) for n = 661691 and 661700.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (661721) by changing a digit.

661720 is an untouchable number, because it is not equal to the sum of proper divisors of any number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 2724 + ... + 2956.

It is an arithmetic number, because the mean of its divisors is an integer number (47385).

2⁶⁶¹⁷²⁰ is an apocalyptic number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 661720, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (758160).

661720 is an abundant number, since it is smaller than the sum of its proper divisors (854600).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

661720 is a wasteful number, since it uses less digits than its factorization.

661720 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 315 (or 311 counting only the distinct ones).

The product of its (nonzero) digits is 504, while the sum is 22.

The square root of 661720 is about 813.4617384979. The cubic root of 661720 is about 87.1414442893.

Adding to 661720 its reverse (27166), we get a palindrome (688886).

The spelling of 661720 in words is "six hundred sixty-one thousand, seven hundred twenty".