I was recently asked who my favorite vampire was. I said, "the muppet from Sesame Street."
They told me, "he doesn't count!" I replied, "I assure you he does."
-Anonymous
Monday, October 26, 2020
Monday, October 12, 2020
Well Yes, But Actually No. Matrix Multiplication Gone Horribly Right.
Saw a funny cartoon meme that showed how to do matrix multiplication the "wrong" way.
It made me wonder how many solutions would work using the wrong method the meme outlined. To answer this, I wrote a quick Ruby script to check all possible values for a1, a2 .. d1, d2 in the range 0-9 to see which satisfy the following matrix multiplication:
| a1 b1 | | a2 b2 | = | a3 b3 | | c1 d1 | | c2 d2 | | c3 d3 |
where:
a3 = a1 * 10 + a2
etc.
For example:
| 3 6 | | 9 3 | = | 39 63 | | 4 3 | | 2 9 | | 42 39 |
or lexically:
| s t | | w x | = | sw tx | | u v | | y z | | uy yz |
Assuming we check every possible combination, there would 9^8, or 43,046,721 combinations. That's not too large, and with some early checks, we should be able to significantly prune the search space down to around 100K.
Ignoring the trivial case (all zeroes), there are exactly 100 possible solutions. Of those, 82 have identical "right" diagonals where:
b1 = b2 and c1 = c2
but only 24 have identical "left" diagonals where:
a1 = a2 and d1 = d2
Some questions to think about:
1) Why exactly 100 solutions? It seems too non-random.
2) Why do so many (82) have matching "right" diagonals?
3) Are some solutions "duplicates" because of rotations, transposition, etc.?
4) Would you see similar numbers for 3x3 matrices?
1) Why exactly 100 solutions? It seems too non-random.
2) Why do so many (82) have matching "right" diagonals?
3) Are some solutions "duplicates" because of rotations, transposition, etc.?
4) Would you see similar numbers for 3x3 matrices?
| a b c | | j k l | = | aj bk cl | | d e f | | m n o | | dm en fo | | g h i | | p q r | | gp hq ir |
For those interested, here is the Ruby code I used:
#!/usr/bin/env ruby
count = 0
ldiags = 0
rdiags = 0
(1..9).each do |a1|
(1..9).each do |b1|
(1..9).each do |c1|
(1..9).each do |d1|
(1..9).each do |a2|
a3x = (a1 * 10) + a2 # expected a3
# actual a3 = (a1 * a2) + (b1 * c2)
# so if (a1 * a2) is already larger
# than our expected (a3x), we can
# skip checking anything else
next if (a1 * a2) > a3x
(1..9).each do |b2|
b3x = (b1 * 10) + b2 # expected b3
# actual b3 = (a1 * b2) + (b1 * d2)
# so if (a1 * b2) is already larger
# than our expected (b3x), we can
# skip checking anything else
next if (a1 * b2) > b3x
(1..9).each do |c2|
a3 = (a1 * a2) + (b1 * c2) # actual a3
next if a3 != a3x
c3x = (c1 * 10) + c2 # expected c3
c3 = (c1 * a2) + (d1 * c2) # actual c3
next if c3 != c3x
(1..9).each do |d2|
b3 = (a1 * b2) + (b1 * d2) # actual b3
next if b3 != b3x
d3x = (d1 * 10) + d2 # expected d3
d3 = (c1 * b2) + (d1 * d2) # actual d3
next if d3 != d3x
# found a solution
count += 1
puts count
puts "| #{a1} #{b1} | | #{a2} #{b2} | = | #{a3} #{b3} |\n"
puts "| #{c1} #{d1} | | #{c2} #{d2} | | #{c3} #{d3} |\n"
if (a1 == a2) && (d1 == d2)
ldiags += 1
end
if (b1 == b2) && (c1 == c2)
rdiags += 1
end
end
end
end
end
end
end
end
end
puts "count: #{count}"
puts "ldiags: #{ldiags}"
puts "rdiags: #{rdiags}"
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