Missing numbers in triangular sequence

Question

I am not sure if this is an appropriate puzzle for this site, but I searched and found 1 or 2 other types of puzzles like this. This is an original puzzle.

Find the missing numbers in rows 8 through 11 of this triangular array:

1
1	1
-3	2	1
15	-7	3	1
-105	40	-12	4	1
945	-315	78	-18	5	1
-10395	3150	-693	132	-25	6	1
?	-38115	7749	-1317	205	-33	7	1
-2027025	?	-103950	16416	-2280	300	-42	8	1
34459425	-8783775	?	-243540	31470	-3690	420	-52	9	1
-654729075	160810650	-28783755	4169880	?	55980	-5670	568	-63	10	1

Answer 1

I have an answer but you're not going to like it. Write $a_{nm}$ for the entry in row n, column m. The 1 at top left is (0,0). Then $a_{nm}$ is

1 if $m=n$ and otherwise is
$(-1)^{m+n+1}(2(n-m)-1)!!\sum_{k=1}^{n-m}\frac{\binom{n-m-1}{k-1}\binom{m+1}{k}}{2k-1}$.

Here

$n!!=n(n-2)\ldots$ where the product stops just before the first nonpositive number (which for odd $n$ as here is the same as "stops at 1"). Instead of $(2(n-m)-1)!!$ we could write $\frac{2(n-m)!}{2^{n-m}(n-m)!}$.

Accordingly, the missing numbers (left to right) are

135135, 540540, 1621620, -513150.

I am dissatisfied with this because

I have this annoying special case for when m=n; the alternating signs have been shoved in by force rather than falling out naturally; the answer is a sum rather than something more explicit; there is no obvious combinatorial interpretation. I expect there is a nicer closed form that makes it more apparent what is actually going on here.

It feels as if

there's some sort of multinomial thing going on here -- imagine dividing $n-1$ things into a set of $m$ and a set of $n-1-m$; then we're choosing $k$ things from one of those sets and $k-1$ from the other; perhaps we should somehow be re-expressing this in terms of first choosing $2k-1$ things and then splitting them up somehow, which might let us get rid of the annoying $2k-1$ in the denominator via the usual binomial-coefficient identities.

Answer 2

Partial answer

Writing the 3rd row of the numbers: -3, -7, -12, -18, ...., letting the starting number be $\epsilon$, the difference of the first and second number in the series be $\lambda$, the difference of difference be $\delta$, ignoring the negative signs, we get:

+-------------+-----+-----+----+
| n-th series |  ϵ  |  λ  | δ  |
+-------------+-----+-----+----+
|           3 |   3 |   4 |  1 |
|           4 |  15 |  25 |  3 |
|           5 | 105 | 105 | 63 |
+-------------+-----+-----+----+

The question is:

Will this help?

Answer 3

The numbers in the first column are: $1, 1, -3, 15, -105, 945, ...$
Except at the beginning, these alternate in sign and are found by multiplying all the odd numbers up to a certain point. $1, 1, -1*3, 1*3*5, -1*3*5*7, 1*3*5*7*9, ...$
Like @Gareth-McCaughan points out, these have a notation using the double factorial symbol. $(-1)!!=-1$, so the first number is $-(-1)!!$.
The second number is $1!!$, the third is $-3!!$, etc. There are some other patterns that can be spotted on the diagonals such as all the 1's on the first diagonal.
The second and third diagonal also seem to follow a simple pattern. The key to finding all the other numbers is the diagonals.
A pattern can be seen if you take repeated differences.

The first diagonal is: $1, 1, 1, 1, 1, 1, ...$
the differences between terms in this sequence are: $0, 0, 0, 0, ...$

The second diagonal is: $1, 2, 3, 4, 5, ...$
and the differences between successive terms are: $1, 1, 1, 1, 1, ...$
the differences of the differences (i.e. the second differences) are: $0, 0, 0, 0, 0, ...$


The third diagonal is: $-3, -7, -12, -18, -25, -33, ...$
the differences are: $-4, -5, -6, -7, -8, ...$
the second differences are: $-1, -1, -1, -1, -1, ...$
the third differences are: $0, 0, 0, 0, 0, ...$


The fourth diagonal is: $15, 40, 78, 132, 205, ...$
differences: $25, 38, 54, 72, ...$
That doesn't look so promising, but maybe if we take the differences again?
second differences: $13, 16, 19, 22, ...$
That looks better; the successive terms differ by 3.
third differences: $3, 3, 3, 3, 3, ...$
fourth differences: $0, 0, 0, 0, 0,...$

The numbers are getting bigger, but there seems to be something to this. Let's keep pressing on.

The fifth diagonal is: $-105, -315, -693, -1317, -2280, -3690,...$
differences: $-210, -378, -624, -963, -1410,...$
second differences: $-168, -246, -339, -447, ...$
third differences: $-78, -93, -108, ...$
fourth differences: $-15, -15, -15, -15, ...$
fifth differences: $0, 0, 0, 0, 0, ...$

Maybe in the sixth diagonal, the sixth differences will all be 0? Let's see.
$945, 3150, 7749, 16416, 31470, 55980, 93870,...$
differences: $2205, 4599, 8667, 15054, 24510, 37890, ...$
second differences: $2394, 4068, 6387, 9456, 13380, ...$
third differences: $1674, 2319, 3069, 3924, ...$
fourth differences: $645, 750, 855, ...$
fifth differences: $105, 105, 105, ....$
sixth differences: $0, 0,0,0,0,...$

So, that's the pattern.

In the $k^{th}$ diagonal, the $k^{th}$ successive differences are 0. The $(k-1)^{th}$ differences in the cases calculated above (for diagonals 3 through 6) were: $-1, 3, -15, 105$. Look familiar? These are the products of the odd numbers again just like in the first column! They have to be those numbers to make the pattern of the differences and everything else work out. Knowing everything about the differences now, you can work backwards to find out what all the original numbers in the triangle have to be.

All those calculations made me hungry. Luckily, my wife is making the world's best spaghetti using a recipe she and her friend stole from some rich guy in a local mansion. It's a funny story how they got the recipe.