A PCR threshold cycle is the first checked count that reaches the threshold. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The finite doubling table is the source
Start with 3 copies. The checked counts are 3,6,12,24,48.
3,6,12,24,48
Cycle three is the first crossing
The threshold is 24. Cycle 2 is below it, and cycle 3 reaches it.