This revised post follows up on harmonic addition mentioned in a previous post *here*.

Harmonic arithmetic is an inverse arithmetic. It is based on an automorphism that interchanges the zero with the infinite and the greater-than-one with the less-than-one: 0 ↔ ∞ and *x* ↔ 1/*x*. So zero becomes the new inaccessible number and infinity becomes the new additive unit. That is,

*x* ⨁ *y* := *g*^{−1}(*g*(*x*) + *g*(*y*))

*x* ⨀ *y* := *g*^{−1}(*g*(*x*) ∙ *g*(*y*))

etc., where *g*(*x*) = 1/*x* with x ≠ 0.

Regular and harmonic arithmetic are isomorphic with ordinary addition exchanged with harmonic multiplication and ordinary multiplication exchanged with harmonic division. Harmonic arithmetic is like counting down from infinity, in which an increment of one reduces the amount slightly.

Harmonic addition is defined as a power operation:

with the understanding that (1/0) → ∞ and (1/∞) → 0. An intermediate value may be zero but not a final value.

Simple harmonic addition is thus defined as:

The harmonic additive unit is infinity instead of zero: x ⊕ ∞ = ∞. Harmonic increment is:

Harmonic subtraction is defined as:

Harmonic multiplication is defined as:

Harmonic division is defined then as:

Harmonic exponentiation is defined as: