# Enigmatic Code

Programming Enigma Puzzles

## Enigma 1391: Platonic puzzle

From New Scientist #2551, 13th May 2006

For homework Penny had to make an icosahedron from 20 equilateral triangles. Each triangle had to be numbered 1, 2, 3, 4 or 5 and at each vertex of the icosahedron the five triangular faces that met there all had to be different numbers.

Joe and Penny each made a model satisfying these requirements and in both cases the faces round the “top” vertex were 1 2 3 4 & 5 clockwise.

Although the overall arrangements of the numbers were different for both models, the orders of the numbers around the “bottom” vertex (diametrically opposite the top vertex) were the same.

Looking at the bottom vertex, what was that order, clockwise starting at 1?

[enigma1391]

### One response to “Enigma 1391: Platonic puzzle”

1. Jim Randell 17 September 2013 at 9:22 am

This Python program runs in 33ms.

```#   /\    /\    /\    /\    /\
#  / 0\  / 1\  / 2\  / 3\  / 4\
# /____\/____\/____\/____\/____\
# \    /\    /\    /\    /\    /\
#  \ 5/10\ 6/11\ 7/12\ 8/13\ 9/14\
#   \/____\/____\/____\/____\/____\
#    \    /\    /\    /\    /\    /
#     \15/  \16/  \17/  \18/  \19/
#      \/    \/    \/    \/    \/

from enigma import printf

# the faces (clockwise around each vertex)
v2f = [
# "top" vertex
(0, 4, 3, 2, 1),
# vertices surrounding "top"
(0, 1, 6, 10, 5),
(1, 2, 7, 11, 6),
(2, 3, 8, 12, 7),
(3, 4, 9, 13, 8),
(4, 0, 5, 14, 9),
# vertices surrounding "bottom"
(5, 10, 15, 19, 14),
(6, 11, 16, 15, 10),
(7, 12, 17, 16, 11),
(8, 13, 18, 17, 12),
(9, 14, 19, 18, 13),
# "bottom" vertex
(15, 16, 17, 18, 19),
]

# colours
colours = list('12345')

# return an updated list
def update(cs, i, c):
cs = list(cs)
cs[i] = c
return cs

# colour the icosohedron
def solve(cs):
# find an uncoloured face
try:
i = cs.index(None)
except ValueError:
# if there are none, so we're done
printf("[{cs}]", cs=' '.join(cs))
# find the colouring around the bottom vertex
r = list(cs[i] for i in v2f[-1])
# and rotate it until 1 is at the start
while r[0] != '1': r.insert(0, r.pop())
print(' '.join(r))
else:
# colour it such that no vertex has duplicate colours
for c in colours:
if any(c in (cs[i] for i in f) for f in v2f if i in f): continue
solve(update(cs, i, c))

# initial configuration
cs = [None] * 20
for (i, c) in zip(v2f[0], colours):
cs[i] = c

solve(cs)
```

Solution: The numbers surrounding the bottom vertex are: 1, 5, 4, 3, 2 (in clockwise order).