# Problem of the Month (August 2007)

In October 1999, we investigated stacking vertical squares. This month we investigate 2 problems involving stacking horizontal squares.

1. Assume we have squares with sides 1, 2, 3, . . . n of fixed thickness 1. By stacking them flat, what is the largest connected area Ak(n) of height k that we can produce? Each square must either be on the lowest level or be fully supported by one or more squares below. What are the values of Ak(n) for large k and n?

2. Assume we have a bunch of cubes of side 1. We want to stack them so that for every i, the boundary of level i+1 is contained within the interior of the boundary of level i. What is the fewest number of cubes Ck(n) that we need to have n cubes on level k? What are the values of Ck(n) for large k and n?

1.

Since the combined area of squares of sides 1 through n is n(n+1)(2n+1)/6, an upper bound is Ak(n) ≤ Sk(n) = n(n+1)(2n+1)/6k.

David Bevan defined the waste w2(n)=2(S2(n)-A2(n)), the area of the lower layer not covered by the upper layer. He showed how to build a cell, 8 consecutive squares placed so that the waste is exactly 16. From this he gets the lower bound A2(n)≥S(n)-n+4 for n≥8.

He improves this by showing how to build a supercell, 40 consecutive squares made from 5 cells with a waste of exactly 48. From this he gets a better lower bound A2(n)≥S2(n)-(3n+48)/5 for n≥40.

Best Known Lower Bounds for Ak(n)
k \ n123456789101112131415
20151021416598138187247318403500611
300151017305274106146
400015101726416589

 A2(1)=0 A2(2)=1 A2(3)=5 A2(4)=10 A2(5)=21 A2(6)=41
 A2(7)≥65 A2(8)≥98(David Cantrell) A2(9)≥138
 A2(10)≥187 A2(11)≥247
 A2(12)≥318(David Bevan) A2(13)≥403(David Bevan)
 A2(14)≥500(David Bevan)
 A2(15)≥611(David Bevan)
 A2(40)≥22116(David Bevan)
 A3(1)=0 A3(2)=0 A3(3)=1 A3(4)=5 A3(5)=10 A3(6)=17 A3(7)=30
 A3(8)≥52 A3(9)≥74 A3(10)≥106
 A3(11)≥146
 A4(1)=0 A4(2)=0 A4(3)=0 A4(4)=1 A4(5)=5 A4(6)=10 A4(7)=17 A4(8)=26
 A4(9)=41 A4(10)≥65 A4(11)≥89

2.

David Cantrell and Berend van der Zwaag sent several solutions.

By building pyramids in which every level is a square, we get the upper bound Ck(n2) ≤ n2 + (n+1)2 + . . . + (n+k-1)2 = kn(n+k-1) + k(k-1)(2k-1)/6.

Best Known Upper Bounds for Ck(n)
k \ n123456789101112
2469111315172022242628
310131921262931
420243335

 C2(1)=4 C2(2)=6Berend vander Zwaag C2(3)≤9 C2(4)≤11Berend vander Zwaag C2(5)≤13David Cantrell
 C2(6)≤15 C2(7)≤17Berend vander Zwaag C2(8)≤20David Cantrell C2(9)≤22

 C2(10)≤24 C2(11)≤26 C2(12)≤28

 C3(1)≤10David Cantrell C3(2)≤13David Cantrell C3(3)≤19Berend vander Zwaag C3(4)≤21Berend vander Zwaag

 C3(5)≤26David Cantrell C3(6)≤29David Cantrell C3(7)≤31

 C4(1)≤20Berend vander Zwaag C4(2)≤24Berend vander Zwaag C4(4)≤35

Claudio Baiocchi thought this problem would be interesting for equilateral triangles, tans, hexagons, and other shapes that tile the plane. Here are the best known solutions for tans Tk(n). Is it possible that T2(n)=2n+1 for all n?

Best Known Upper Bounds for Tk(n)
k \ n123456
235791113
3691215
41014

 T2(1)=3 T2(2)=5 T2(3)=7 T2(4)=9 T2(5)=11 T2(6)=13

 T3(1)=6 T3(2)=9 T3(3)=12David Cantrell T3(4)=15David Cantrell

 T4(1)=10David Cantrell T4(2)=14David Cantrell

If you can extend any of these results, please e-mail me. Click here to go back to Math Magic. Last updated 8/30/07.