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28 = 227

• 28 can be written using four 4's:

See also 113.
28 has 6 divisors (see below), whose sum is σ = 56. Its totient is φ = 12.

The previous prime is 23. The next prime is 29. The reversal of 28 is 82.

It is a happy number.

28 is nontrivially palindromic in base 3, base 6 and base 13.

It is a Cunningham number, because it is equal to 33+1.

28 is an esthetic number in base 8, because in such base its adjacent digits differ by 1.

28 is a nontrivial binomial coefficient, being equal to C(8, 2).

It is a harmonic number, since the harmonic mean of its divisors is an integer.

28 is a repfigit number.

It is a fibodiv number, since the Fibonacci-like sequence with seeds 2 and 8 contains 28 itself.

28 is an idoneal number.

It is an Ulam number.

28 is a nontrivial repdigit in base 6 and base 13.

It is a plaindrome in base 6, base 8, base 10, base 11, base 12, base 13, base 15 and base 16.

It is a nialpdrome in base 2, base 6, base 7, base 9, base 13 and base 14.

It is a zygodrome in base 2, base 6 and base 13.

It is a congruent number.

It is a pernicious number, because its binary representation contains a prime number (3) of ones.

It is an upside-down number.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 1 + ... + 7.

28 is the 7-th triangular number and also the 4-th hexagonal number.

28 is the 3-rd centered nonagonal number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 28, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (28).

28 is a perfect number, since it is equal to the sum of its proper divisors.

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

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

The sum of its prime factors is 11 (or 9 counting only the distinct ones).

The product of its digits is 16, while the sum is 10.

The square root of 28 is about 5.2915026221. The cubic root of 28 is about 3.0365889719.

Adding to 28 its product of digits (16), we get a palindrome (44).

The spelling of 28 in words is "twenty-eight", and thus it is an aban number, an oban number, and an uban number.

Divisors: 1 2 4 7 14 28