Prove the Impossible Lecture 6: Sep 19 (based on slides in MIT 6.042)

Domino Puzzle An 8x8 chessboard, 32 pieces of dominos Can we fill the chessboard?

Domino Puzzle An 8x8 chessboard, 32 pieces of dominos Easy!

Domino Puzzle An 8x8 chessboard with two holes, 31 pieces of dominos Can we fill the chessboard? Easy!

Domino Puzzle An 8x8 chessboard with two holes, 31 pieces of dominos Can we fill the chessboard? Easy??

Domino Puzzle An 4x4 chessboard with two holes, 7 pieces of dominos Can we fill the chessboard? Impossible!

Domino Puzzle An 8x8 chessboard with two holes, 31 pieces of dominos Can we fill the chessboard? Then what??

Another Chessboard Problem ? A rook can only move along a diagonal Can a rook move from its current position to the question mark?

Another Chessboard Problem ? A rook can only move along a diagonal Can a rook move from its current position to the question mark? Impossible! Why?

Another Chessboard Problem ? 1.The rook is in a blue position. 2.A blue position can only move to a blue position by diagonal moves. 3.The question mark is in a white position. 4.So it is impossible for the rook to go there. Invariant! This is a very simple example of the invariant method.

Domino Puzzle An 8x8 chessboard with two holes, 31 pieces of dominos Can we fill the chessboard?

Domino Puzzle 1.Each domino will occupy one white square and one blue square. 2.There are 32 blue squares but only 30 white squares. 3.So it is impossible to fill the chessboard using only 31 dominos. Invariant! This is a simple example of the invariant method.

Invariant Method 1.Find properties (the invariants) that are satisfied throughout the whole process. 2.Show that the target do not satisfy the properties. 3.Conclude that the target is not achievable. In the rook example, the invariant is the colour of the position of the rook. In the domino example, the invariant is that any placement of dominos will occupy the same number of blue positions and white positions.

The Possible We just proved that if we take out two squares of the same colour, then it is impossible to finish. What if we take out two squares of different colours? Would it be always possible to finish then? Yes??

Prove the Possible Yes??

Prove the Possible The secret.

Prove the Possible The secret.

Fifteen Puzzle Move: can move a square adjacent to the empty square to the empty square.

Fifteen Puzzle Initial configuration Target configuration Is there a sequence of moves that allows you to start from the initial configuration to the target configuration?

Invariant Method 1.Find properties (the invariants) that are satisfied throughout the whole process. 2.Show that the target do not satisfy the properties. 3.Conclude that the target is not achievable. What is an invariant in this game?? This is usually the hardest part of the proof.

Hint Initial configuration Target configuration ((1,2,3,…,14,15),(4,4)) ((1,2,3,…,15,14),(4,4)) Hint: the two states have different parity.

Parity Given a sequence, a pair is “out-of-order” if the first element is larger. For example, the sequence (1,2,4,5,3) has two out-of-order pairs, (4,3) and (5,3). Given a state S = ((a1,a2,…,a15),(i,j)) Parity of S = (number of out-of-order pairs + i) mod 2 row number of the empty square

Hint Initial configuration Target configuration ((1,2,3,…,14,15),(4,4)) ((1,2,3,…,15,14),(4,4)) Clearly, the two states have different parity. Parity of S = (number of out-of-order pairs + i) mod 2

Invariant Method 1.Find properties (the invariants) that are satisfied throughout the whole process. 2.Show that the target do not satisfy the properties. 3.Conclude that the target is not achievable. Invariant = parity of state Claim: Any move will preserve the parity of the state. Proving the claim will finish the impossibility proof. Parity is even Parity is odd

Proving the Invariant Claim: Any move will preserve the parity of the state. Parity of S = (number of out-of-order pairs + i) mod 2 ???? ?a? ???? ???? ???? ?a? ???? ???? Horizontal movement does not change anything…

Proving the Invariant Claim: Any move will preserve the parity of the state. Parity of S = (number of out-of-order pairs + i) mod 2 ???? ?ab1b2 b3?? ???? ???? ?b1b2 b3a?? ???? If there are (0,1,2,3) out-of-order pairs in the current state, there will be (3,2,1,0) out-of-order pairs in the next state. Row number has changed by 1 So the parity stays the same! We’ve proved the claim. Difference is 1 or 3.

Fifteen Puzzle Initial configuration Target configuration Is there a sequence of moves that allows you to start from the initial configuration to the target configuration? This is a standard example of the invariant method.

Checker x=0 Start with any configuration with all men on or below the x-axis.

Checker x=0 Move: jump through your adjacent neighbour, but then your neighbour will disappear.

Checker x=0 Move: jump through your adjacent neighbour, but then your neighbour will disappear.

Checker x=0 Goal: Find an initial configuration with least number of men to jump up to level k.

K=1 x=0 2 men.

K=2 x=0

K=2 x=0 4 men. Now we have reduced to the k=1 configuration, but one level higher.

K=3 x=0 This is the configuration for k=2, so jump two level higher.

K=3 x=0 8 men.

K=4 x=0

K=4 x=0

K=4 x=0

K=4 x=0

K=4 x=0 Now we have reduced to the k=3 configuration, but one level higher 20 men!

K=5 a.39 or below b men c men d men e.101 – 1000 men f.1001 or above None of the above (but f is closest), it is impossible! This is a tricky example of the invariant method. Excellent project idea

Classwork 1 October 3 (in class) 1.Logic. 2.Sets, functions. 3.Proof by cases, contradiction. 4.Proof by inductions. 5.Invariant method. True or false, multiple choice, short question, long question. A very useful method