661721687 has 8 divisors (see below), whose sum is σ = 722545920. Its totient is φ = 601008760.

The previous prime is 661721677. The next prime is 661721729. The reversal of 661721687 is 786127166.

661721687 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 661721687 - 2¹² = 661717591 is a prime.

It is a super-2 number, since 2×661721687² = 875751182092251938, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (661721677) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 15137 + ... + 39402.

It is an arithmetic number, because the mean of its divisors is an integer number (90318240).

Almost surely, 2^661721687 is an apocalyptic number.

661721687 is a deficient number, since it is larger than the sum of its proper divisors (60824233).

661721687 is a wasteful number, since it uses less digits than its factorization.

661721687 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 55653.

The product of its digits is 169344, while the sum is 44.

The square root of 661721687 is about 25723.9516210088. The cubic root of 661721687 is about 871.4151834252.

The spelling of 661721687 in words is "six hundred sixty-one million, seven hundred twenty-one thousand, six hundred eighty-seven".