661651 has 4 divisors (see below), whose sum is σ = 668304. Its totient is φ = 655000.

The previous prime is 661621. The next prime is 661663. The reversal of 661651 is 156166.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 661651 - 2⁹ = 661139 is a prime.

It is an Ulam number.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (661621) 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, 3175 + ... + 3376.

It is an arithmetic number, because the mean of its divisors is an integer number (167076).

2⁶⁶¹⁶⁵¹ is an apocalyptic number.

661651 is a deficient number, since it is larger than the sum of its proper divisors (6653).

661651 is a wasteful number, since it uses less digits than its factorization.

661651 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 6652.

The product of its digits is 1080, while the sum is 25.

The square root of 661651 is about 813.4193260551. The cubic root of 661651 is about 87.1384153301.

It can be divided in two parts, 66165 and 1, that added together give a palindrome (66166).

The spelling of 661651 in words is "six hundred sixty-one thousand, six hundred fifty-one".