Stellar parallax converts a tiny measured angular shift into a distance through d = one over p, with p in arcseconds giving d in parsecs. The arithmetic is exact, but p itself is a measured quantity with finite precision, and because d is inversely proportional, the fractional error in p propagates directly into the inferred distance. The inverse law is the model; the angle is the only true input.
A normalized parallax index is checked by its inverse distance index. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
A parallax index has an inverse distance index
The parallax index is 1/5. The checked distance index is 5.
p=51,d=5
The product is the source check
The product 1 confirms that this is a normalized inverse-index rule, not a parsec calibration.