• 28 can be written using four 4's:
It is a happy number.
28 is nontrivially palindromic in base 3, base 6 and base 13.
28 is an esthetic number in base 8, because in such base its adjacent digits differ by 1.
It is a harmonic number, since the harmonic mean of its divisors is an integer.
28 is a repfigit number.
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 an upside-down number.
28 is the 3-rd centered nonagonal number.
It is an amenable number.
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 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).