51661 has 4 divisors (see below), whose sum is σ = 54400. Its totient is φ = 48924.

The previous prime is 51659. The next prime is 51673. The reversal of 51661 is 16615.

51661 is an esthetic number in base 3, because in such base its adjacent digits differ by 1.

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, and also an emirpimes, since its reverse is a distinct semiprime: 16615 = 5 ⋅3323.

It is a cyclic number.

It is not a de Polignac number, because 51661 - 2¹ = 51659 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (19), and also a Moran number because the ratio is a prime number: 2719 = 51661 / (5 + 1 + 6 + 6 + 1).

It is a Duffinian number.

It is a nialpdrome in base 9.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (51631) 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, 1341 + ... + 1378.

It is an arithmetic number, because the mean of its divisors is an integer number (13600).

2⁵¹⁶⁶¹ is an apocalyptic number.

It is an amenable number.

51661 is a deficient number, since it is larger than the sum of its proper divisors (2739).

51661 is a wasteful number, since it uses less digits than its factorization.

51661 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2738.

The product of its digits is 180, while the sum is 19.

The square root of 51661 is about 227.2905629365. The cubic root of 51661 is about 37.2438243294.

The spelling of 51661 in words is "fifty-one thousand, six hundred sixty-one".