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If we rewrite the matrix putting on the top the upper left edge and then we sum the numbers following a boustrophedon path we have:
.................................1.\
...............................____|
............................../1..1
............................./_______
...........................1...2...1.\
..........................____________\
........................./.1...2...1...1.
......................../________________
........................1...2...3...2...1.\
......................_____________________\
......................1...2...3...1...1...1
1, 1+1, 1+1+1, 1+1+1+1, 1+1+1+1+2, 1+1+1+1+2+1, 1+1+1+1+2+1+1, 1+1+1+1+2+1+1+1, 1+1+1+1+2+1+1+1+2, 1+1+1+1+2+1+1+1+2+1, etc.
That is 1, 2, 3, 4, 6, 7, 8, 9, 11, 12, 13, 15, 18, 20, etc.
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