# Problem of the Month (June 2017)

Consider graphs where each vertex is labeled with a number. We are interested in planar graphs where the sum of the vertices adjacent to n is 3n–1. (The case where the sum for every vertex is 3n is more natural, but then any 3-regular graph could be labeled with any one number.) What is the smallest such graph (by number of vertices, and sum of vertices as a tiebreaker) whose largest digit is n? Can you prove that such graphs always exist? What if the sum of vertices is 3n+1, 4n–1, 4n+1, 5n–1 or 5n+1?

George Sicherman confirmed that all the 3n–1 and 3n+1 graphs below are optimal.

Here are the best known solutions.

3n–1 Solutions
12345678910
11121314151617181920

(George Sicherman)
21222324252627282930

(Bryce Herdt)
31323334353637383940

(George Sicherman)

(Joe DeVincentis)
41424344454647484950

(George Sicherman)

(George Sicherman)

(Joe DeVincentis)

3n+1 Solutions
12345678910

(Bryce Herdt)

(George Sicherman)
11121314151617181920

(Bryce Herdt)

(George Sicherman)

(Bryce Herdt)
21222324252627282930

(George Sicherman)

(George Sicherman)

(Bryce Herdt)

(George Sicherman)
31323334353637383940

(George Sicherman)

(Bryce Herdt)
41424344454647484950

(George Sicherman)

(George Sicherman)

(George Sicherman)

4n–1 Solutions
1234567
891011121314
15161718192021

(George Sicherman)

(George Sicherman)

(George Sicherman)

(Joe DeVincentis)

(George Sicherman)
22232425262728

(George Sicherman)

(George Sicherman)

(George Sicherman)

(George Sicherman)
29303132333435

(Joe DeVincentis)

(George Sicherman)

(George Sicherman)

(George Sicherman)

(George Sicherman)
36373839404142

(Bryce Herdt)

(George Sicherman)

(Joe DeVincentis)
43444546474849

(George Sicherman)

(Joe DeVincentis)

(George Sicherman)

(George Sicherman)

(George Sicherman)
50

4n+1 Solutions
123456

(George Sicherman)

(George Sicherman)

(George Sicherman)

(George Sicherman)
789101112

(George Sicherman)

(George Sicherman)
131415161718

(George Sicherman)

(George Sicherman)

(Joe DeVincentis)

(George Sicherman)
192021222324

(George Sicherman)

(George Sicherman)
252627282930

(George Sicherman)

(Joe DeVincentis)
313233343536

(George Sicherman)

(George Sicherman)

(George Sicherman)
373839404142

(Joe DeVincentis)

(George Sicherman)
434445464748

(George Sicherman)
4950

(Joe DeVincentis)

5n–1 Solutions
12345

(Joe DeVincentis)

(Joe DeVincentis)
678910

(Bryce Herdt)

(Bryce Herdt)

(Joe DeVincentis)
1112131415

(George Sicherman)

(Joe DeVincentis)

(Bryce Herdt)

(Joe DeVincentis)

(George Sicherman)
1617181920

(George Sicherman)

(Joe DeVincentis)

(George Sicherman)

(George Sicherman)
2122232425

(George Sicherman)

(Joe DeVincentis)

(George Sicherman)

(George Sicherman)
2627282930

(George Sicherman)

(Joe DeVincentis)

(Joe DeVincentis)

(Bryce Herdt)

(George Sicherman)
3132333435

(Joe DeVincentis)

(Joe DeVincentis)

(George Sicherman)

(Joe DeVincentis)

(Joe DeVincentis)
3637383940

(George Sicherman)

(George Sicherman)

(George Sicherman)

(George Sicherman)

(George Sicherman)
4142434445

(Joe DeVincentis)

(Joe DeVincentis)

(George Sicherman)

(George Sicherman)

(George Sicherman)
4647484950

(George Sicherman)

(Joe DeVincentis)

(George Sicherman)

(George Sicherman)

(George Sicherman)

5n+1 Solutions
1234
none
(Bryce Herdt)

(Bryce Herdt)

(Bryce Herdt)
5678

(Joe DeVincentis)

(Joe DeVincentis)

(George Sicherman)
9101112

(George Sicherman)

(Bryce Herdt)

(Joe DeVincentis)

(George Sicherman)
13141516

(Bryce Herdt)

(George Sicherman)

(George Sicherman)

(George Sicherman)
17181920

(George Sicherman)

(Joe DeVincentis)

(Bryce Herdt)

(Joe DeVincentis)
21222324

(Joe DeVincentis)

(Joe DeVincentis)

(Joe DeVincentis)

(Joe DeVincentis)
25262728

(Joe DeVincentis)

(Joe DeVincentis)

(George Sicherman)

(Joe DeVincentis)
29303132

(Joe DeVincentis)

(George Sicherman)

(Bryce Herdt)

(Joe DeVincentis)
33343536

(Joe DeVincentis)

(Joe DeVincentis)

(Joe DeVincentis)

(George Sicherman)
37383940

(Joe DeVincentis)

(Joe DeVincentis)

(George Sicherman)

(George Sicherman)
41424344

(Joe DeVincentis)

(Joe DeVincentis)

(Joe DeVincentis)

(Joe DeVincentis)
45464748

(George Sicherman)

(Joe DeVincentis)

(George Sicherman)

(Joe DeVincentis)
4950

(Joe DeVincentis)

(George Sicherman)

6n–1 Solutions
12345

(George Sicherman)

(George Sicherman)

(George Sicherman)

(George Sicherman)

(George Sicherman)

Joe DeVincentis showed that there are 3n–1 solutions for every n, by generalizing the graphs below:

Bryce Herdt showed that there are 5n–1 solutions for every n. Start with a square anti-prism of n's. Build rings of n–1's to 1's outward from the outer ring, and inward from the inner ring. Connect each k to two k–1's (only one when k=n, and none when k=1). The first two graphs in this sequence are shown below:

This construction can be modified to give a 4n–1 solution for every n. Omit one edge from every vertex to a similarly labeled vertex.

If you can extend any of these results, please e-mail me. Click here to go back to Math Magic. Last updated 6/1/17.