• 78 can be written using four 4's:

See also 113.

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78 has 8 divisors (see below), whose sum is σ = 168. Its totient is φ = 24.

The previous prime is 73. The next prime is 79. The reversal of 78 is 87.

78 is nontrivially palindromic in base 5, base 7 and base 12.

78 is digitally balanced in base 4, because in such base it contains all the possibile digits an equal number of times.

78 is an esthetic number in base 6 and base 10, because in such bases its adjacent digits differ by 1.

78 is a nontrivial binomial coefficient, being equal to C(13, 2).

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

78 is an admirable number.

78 is a Gilda number.

78 is an idoneal number.

It is an alternating number because its digits alternate between odd and even.

It is a house number.

78 is an undulating number in base 5 and base 7.

78 is strictly pandigital in base 4.

It is a Curzon number.

78 is a nontrivial repdigit in base 12.

It is a plaindrome in base 8, base 10, base 12, base 14 and base 16.

It is a nialpdrome in base 3, base 6, base 9, base 11, base 12, base 13 and base 15.

It is a zygodrome in base 12.

It is a congruent number.

It is a panconsummate number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1 + ... + 12.

It is an arithmetic number, because the mean of its divisors is an integer number (21).

78 is the 12-th triangular number.

It is a practical number, because each smaller number is the sum of distinct divisors of 78, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (84).

78 is a primitive abundant number, since it is smaller than the sum of its proper divisors, none of which is abundant.

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

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

With its predecessor (77) it forms a Ruth-Aaron pair, since the sum of their distinct prime factors is the same (18).

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

The sum of its prime factors is 18.

The product of its digits is 56, while the sum is 15.

The square root of 78 is about 8.8317608663. The cubic root of 78 is about 4.2726586817.

Subtracting from 78 its product of digits (56), we obtain a palindrome (22).

Multiplying 78 by its reverse (87), we get a triangular number (6786 = T₁₁₆).

The spelling of 78 in words is "seventy-eight", and thus it is an aban number, an oban number, and an uban number.