1666781 has 2 divisors, whose sum is σ = 1666782. Its totient is φ = 1666780.

The previous prime is 1666771. The next prime is 1666783. The reversal of 1666781 is 1876661.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 1666681 + 100 = 1291^2 + 10^2 .

It is a cyclic number.

It is not a de Polignac number, because 1666781 - 2¹⁰ = 1665757 is a prime.

It is a super-2 number, since 2×1666781² = 5556317803922, which contains 22 as substring.

Together with 1666783, it forms a pair of twin primes.

It is a Chen prime.

It is an Ulam number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (1666783) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 833390 + 833391.

It is an arithmetic number, because the mean of its divisors is an integer number (833391).

2^1666781 is an apocalyptic number.

It is an amenable number.

1666781 is a deficient number, since it is larger than the sum of its proper divisors (1).

1666781 is an equidigital number, since it uses as much as digits as its factorization.

1666781 is an evil number, because the sum of its binary digits is even.

The product of its digits is 12096, while the sum is 35.

The square root of 1666781 is about 1291.0387290860. The cubic root of 1666781 is about 118.5658212308.

The spelling of 1666781 in words is "one million, six hundred sixty-six thousand, seven hundred eighty-one".