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Index Crash Course
NORMAL SUDOKU
– Every row, column, and box must contain the digits 1-4 with no repeats
DOPPELGANGER
– Every row, column, and box contains the digit 0 always, as well as three out of the four digits from 1 to 4 (ie, it's missing one digit that is not 0)
– No two row, no two column, and no two box may be missing the same digit
– For each 0 in the grid, the digits missing in its row, its column, and its box must be three different digits (and note that a zero always acts as itself – ie, the lowest possible value – regardless of what digits it's replacing in its region, row and column)
INDEX LINES
– Each cell on an index line has a position number: the square is position 1 for Normal Sudoku and 0 for Doppelganger, and it counts up from there
– If a digit with value V is at position P, then the digit with value P must be at position V
– Every row, column, and box must contain the digits 1-4 with no repeats
DOPPELGANGER
– Every row, column, and box contains the digit 0 always, as well as three out of the four digits from 1 to 4 (ie, it's missing one digit that is not 0)
– No two row, no two column, and no two box may be missing the same digit
– For each 0 in the grid, the digits missing in its row, its column, and its box must be three different digits (and note that a zero always acts as itself – ie, the lowest possible value – regardless of what digits it's replacing in its region, row and column)
INDEX LINES
– Each cell on an index line has a position number: the square is position 1 for Normal Sudoku and 0 for Doppelganger, and it counts up from there
– If a digit with value V is at position P, then the digit with value P must be at position V