1661 has 4 divisors (see below), whose sum is σ = 1824. Its totient is φ = 1500.

The previous prime is 1657. The next prime is 1663.

1661 is nontrivially palindromic in base 10.

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 a cyclic number.

It is not a de Polignac number, because 1661 - 2² = 1657 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a plaindrome in base 9, base 13 and base 16.

It is a nialpdrome in base 12.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1663) 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, 65 + ... + 86.

It is an arithmetic number, because the mean of its divisors is an integer number (456).

2¹⁶⁶¹ is an apocalyptic number.

1661 is a gapful number since it is divisible by the number (11) formed by its first and last digit.

It is an amenable number.

1661 is a deficient number, since it is larger than the sum of its proper divisors (163).

1661 is a wasteful number, since it uses less digits than its factorization.

1661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 162.

The product of its digits is 36, while the sum is 14.

The square root of 1661 is about 40.7553677446. The cubic root of 1661 is about 11.8428586049.

It can be divided in two parts, 16 and 61, that added together give a palindrome (77).

The spelling of 1661 in words is "one thousand, six hundred sixty-one".