• 861 can be written using four 4's:

See also 113.

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861 has 8 divisors (see below), whose sum is σ = 1344. Its totient is φ = 480.

The previous prime is 859. The next prime is 863. The reversal of 861 is 168.

861 is nontrivially palindromic in base 6.

861 is a nontrivial binomial coefficient, being equal to C(42, 2).

It is an interprime number because it is at equal distance from previous prime (859) and next prime (863).

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

It is not a de Polignac number, because 861 - 2¹ = 859 is a prime.

It is a Smith number, since the sum of its digits (15) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is an Ulam number.

It is a Curzon number.

It is a plaindrome in base 9, base 14 and base 16.

It is a nialpdrome in base 10.

It is a self number, because there is not a number n which added to its sum of digits gives 861.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 861.

It is not an unprimeable number, because it can be changed into a prime (863) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 1 + ... + 41.

It is an arithmetic number, because the mean of its divisors is an integer number (168).

2⁸⁶¹ is an apocalyptic number.

861 is the 41-st triangular number and also the 21-st hexagonal number.

It is an amenable number.

861 is a deficient number, since it is larger than the sum of its proper divisors (483).

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

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

The sum of its prime factors is 51.

The product of its digits is 48, while the sum is 15.

The square root of 861 is about 29.3428015022. The cubic root of 861 is about 9.5133699099.

Adding to 861 its product of digits (48), we get a palindrome (909).

Multiplying 861 by its product of digits (48), we get a triangular number (41328 = T₂₈₇).

The spelling of 861 in words is "eight hundred sixty-one", and thus it is an aban number.