Spin echo visibility grows only when the recoverable bucket grows. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

Recoverable and irreversible buckets trade off inside one budget

Initial transverse budget is 15 and coherent bucket is fixed at 5.

echo visible=coherent+recoverable\text{echo visible}=\text{coherent}+\text{recoverable}
Echo split rowThe echo adds only the recoverable bucket to the coherent bucket.coherent 5recoverable 4irreversible 6echo 9

Three split rows keep the initial budget closed

The irreversible bucket stays outside the echo-visible result.

recoverablelostechoFecho2877154693564111115\begin{array}{c|c|c|c}\text{recoverable}&\text{lost}&\text{echo}&F_{\text{echo}}\\2&8&7&\frac{7}{15}\\4&6&9&\frac{3}{5}\\6&4&11&\frac{11}{15}\\\end{array}

The middle row makes nine visible

Coherent 5 plus recoverable 4 gives echo-visible signal 9.

5+4=95+4=9
Echo split rowThe echo adds only the recoverable bucket to the coherent bucket.coherent 5recoverable 4irreversible 6echo 9