Hexagonal number
A hexagonal number is a figurate number.
The nth hexagonal number hn is the number of distinct dots in a pattern of dots consisting of the outlines of regular hexagons with sides up to n dots, when the hexagons are overlaid so that they share one vertex.
The formula for the nth hexagonal number The first few hexagonal numbers (sequence A000384 in the OEIS) are: Every hexagonal number is a triangular number, but only every other triangular number (the 1st, 3rd, 5th, 7th, etc.)
is a hexagonal number.
Like a triangular number, the digital root in base 10 of a hexagonal number can only be 1, 3, 6, or 9.
The digital root pattern, repeating every nine terms, is "1 6 6 1 9 3 1 3 9".
Every even perfect number is hexagonal, given by the formula The largest number that cannot be written as a sum of at most four hexagonal numbers is 130.
Adrien-Marie Legendre proved in 1830 that any integer greater than 1791 can be expressed in this way.
In addition, only two integers cannot be expressed using five hexagonal numbers (but can be with six), those being 11 and 26.
Hexagonal numbers should not be confused with centered hexagonal numbers, which model the standard packaging of Vienna sausages.
To avoid ambiguity, hexagonal numbers are sometimes called "cornered hexagonal numbers".
One can efficiently test whether a positive integer x is a hexagonal number by computing If n is an integer, then x is the nth hexagonal number.
If n is not an integer, then x is not hexagonal.
The nth number of the hexagonal sequence can also be expressed by using sigma notation as where the empty sum is taken to be 0.
The sum of the reciprocal hexagonal numbers is 2ln(2), where ln denotes natural logarithm.
Using rearrangement, the next set of formulas is given:
Using the final formula from before with respect to m and then n, and then some reducing and moving, one can get to the following equation:
divisors.
Likewise, for any natural number of the form
where p and q are distinct prime numbers,
has divisors of the form
Each combination of k and l yields a distinct divisor, so
∎ The sequence of numbers that are both hexagonal and perfect squares starts 1, 1225, 1413721,... OEIS: A046177.
