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Equation 1 · How Mathematics, Proof, and Scientific Computation Actually Work

What does this equation mean?

fl(xopy)=(xopy)(1+δ),∣δ∣≤u,\mathrm{fl}(x \mathbin{\text{op}} y) = (x \mathbin{\text{op}} y)(1 + \delta), \qquad |\delta| \le u,

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Inputs and operations(x mathbinop y)(1 + delta), qquad |delta| ≤ u
Result or conditionfl(x mathbinop y)
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This equation states a bound: one expression must stay on the indicated side of the other under the article’s assumptions. Read the equation part by part below; each part has a contextual explanation and a link to its mathematical background.

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xx

Symbol x

x is an argument of the function-like quantity on the left; its role is set by that function’s stated inputs.

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yy

Symbol y

y is an argument of the function-like quantity on the left; its role is set by that function’s stated inputs.

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δ\delta

Symbol delta

delta is one of the signed contributions combined to compute the quantity on the left.

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uu

Symbol u

the unit roundoff — roughly 10^{-16} for standard double precision — and op\mathrm{op} is addition, subtraction, multiplication, or division.

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=

=

The expressions on both sides represent the same quantity under the stated assumptions.

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addition

addition

Add the term after the plus sign to the term or group before it.

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How to interpret it

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What the article says around this equation

Computers represent real numbers with a finite number of bits, following a specification — IEEE 754 — that fixes the format and the rounding behavior of arithmetic exactly enough that different compliant hardware and software give reproducible results for the same operations [ 5 ] . The standard’s core guarantee is deceptively narrow: each individual elementary operation is correctly rounded , meaning the result is the closest representable floating-point number to the true mathematical result of that one operation. For a single addition or multiplication, this means fl(xopy)=(xopy)(1+δ),∣δ∣≤u\mathrm{fl}(x \mathbin{\text{op}} y) = (x \mathbin{\text{op}} y)(1 + \delta), \qquad |\delta| \le u. where u is the unit roundoff — roughly 10^{-16} for standard double precision — and op\mathrm{op} is…
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Computers represent real numbers with a finite number of bits, following a specification — IEEE 754 — that fixes the format and the rounding behavior of arithmetic exactly enough that different compliant hardware and software give reproducible results for the same operations [ 5 ] . The standard’s core guarantee is deceptively narrow: each individual elementary operation is correctly rounded , meaning the result is the closest representable floating-point number to the true mathematical result of that one operation. For a single addition or multiplication, this means fl(xopy)=(xopy)(1+δ),∣δ∣≤u\mathrm{fl}(x \mathbin{\text{op}} y) = (x \mathbin{\text{op}} y)(1 + \delta), \qquad |\delta| \le u. where u is the unit roundoff — roughly 10^{-16} for standard double precision — and op\mathrm{op} is addition, subtraction, multiplication, or division. Each single step, in other words, is correct up to a tiny, precisely bounded relative error.

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