Concept
Real numbers
The whole continuum of magnitudes, filling every gap the fractions leave between them.
Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.
The row that is not on the list
Write down a list of infinite sequences, any list at all, and there is a rule that builds a sequence missing from it. The rule reads one entry from each row, and it is the single most reused argument in this field.
The sum that fits in one square
Half, then a quarter, then an eighth, forever. Adding infinitely many things sounds like it should give infinity, and the picture that says otherwise is a square with a corner left uncut.
Named alongside it
The objects these essays reach for when they reach for this one.
BijectionCardinalityConvergenceConvergence rateCountabilityDiagonal argumentDissectionGeometric seriesHarmonic seriesLimitMethod of exhaustionPower series