OFFSET
0,5
COMMENTS
There are 14 ways to put parentheses in the expression a - b - c - d - e: ((a - (b - c)) - d) - e, (((a - b) - c) - d) - e, ((a - b) - (c - d)) - e, etc. This sequence describes how many sets of natural numbers [a,b,c,d,e] can be produced with the numbers {0,1,2,3,...,n} such that the values of all the distinct expressions are different.
It can be shown that in the set of expressions obtained this way, for any number of variables, a is always positive, b is always negative, and the other variables appear with every possible combination of signs. Therefore, the valid k-tuples of numbers in [0..n] are precisely those such that every subset of {c,d,e,...}, including the empty subset, has a distinct sum. For 5 variables, there are n*(n-1)*(n-2) ways to choose distinct, nonzero values for c, d, and e. For each k, there are floor((n-1)/2) ways to choose distinct numbers x and y in [0..n] such that x + y = k. Summing over all k in [0..n], allowing arbitrary permutations of {x,y,k}, and allowing a and b to be any value gives the formula below. - Charlie Neder, Jan 13 2019
LINKS
Charlie Neder, Table of n, a(n) for n = 0..1000
Index entries for linear recurrences with constant coefficients, signature (3,0,-8,6,6,-8,0,3,-1).
FORMULA
a(n) = (n+1)^2 * (n*(n-1)*(n-2) - 6*A002620(n-1)). - Charlie Neder, Jan 13 2019
EXAMPLE
For example, no such sets can be produced with only 0's, only 0's and 1's, only 0's and 1's and 2's, only 1's and 2's and 3's; with {0,1,2,3,4}, 300 such sets can be produced.
PROG
(PARI) a(n) = (1+n)^2*(3*(-1)^n+4*n^3-18*n^2+20*n-3)/4; \\ Jinyuan Wang, Jun 27 2020
CROSSREFS
KEYWORD
nonn,nice,easy
AUTHOR
Asher Auel, Jan 27 2000
EXTENSIONS
a(9)-a(36) from Charlie Neder, Jan 13 2019
Incorrect formula removed by Jinyuan Wang, Jun 27 2020
STATUS
approved