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❈♦♠❜✐♥✐♥❣ ❙✉♣❡r♣♦s✐t✐♦♥ ❛♥❞ ■♥❞✉❝t✐♦♥✿ ❛ Pr❛❝t✐❝❛❧ ❘❡❛❧✐③❛t✐♦♥ ❆❜❞❡❧❦❛❞❡r ❑❡rs❛♥✐ ❛♥❞ ◆✐❝♦❧❛s P❡❧t✐❡r ▲❛❜♦r❛t♦✐r❡ ❞✬■♥❢♦r♠❛t✐q✉❡ ❞❡ ●r❡♥♦❜❧❡✴❈◆❘❙ ❈❆PP t❡❛♠ ✲ ❆❙❆P ♣r♦❥❡❝t ✭❆◆❘✲✵✾✲❇▲❆◆✲✵✹✵✼✲✵✶✮ ❋❘❖❈❖❙ ✷✵✶✸ ✲ ❙❡♣t❡♠❜❡r ✷✵✶✸ ✲ ◆❛♥❝② ❆❜❞❡❧❦❛❞❡r ❑❡rs❛♥✐ ❛♥❞ ◆✐❝♦❧❛s P❡❧t✐❡r ❈♦♠❜✐♥✐♥❣ ❙✉♣❡r♣♦s✐t✐♦♥ ❛♥❞ ■♥❞✉❝t✐♦♥

■♥tr♦❞✉❝t♦r② ❡①❛♠♣❧❡ ❧❡♥❣t❤❴❛t❴❧❡❛st ( l , n ) ⇔ n = ✵ ∨ ∃ x , l ′ , n ′ ( l = cons ( x , l ′ ) ∧ n = s ( n ′ ) ∧ ❧❡♥❣t❤❴❛t❴❧❡❛st ( l ′ , n ′ )) ∃ l ′ l = cons ( y , l ′ ) ∧ ♥t❤ ( x , l , n ) ⇔ ( n = s ( ✵ ) ∧ x = y ) ∨ ∃ n ′ ( n = s ( n ′ ) ∧ ♥t❤ ( x , l ′ , n ′ )) ❈❤❡❝❦ t❤❛t t❤❡ ❢♦❧❧♦✇✐♥❣ ❤♦❧❞s✿ ∀ n ∈ N , ∀ l ( ❧❡♥❣t❤❴❛t❴❧❡❛st ( l , n ) ∧ n � = ✵ ⇒ ∃ x ♥t❤ ( x , l , n )) ❆❜❞❡❧❦❛❞❡r ❑❡rs❛♥✐ ❛♥❞ ◆✐❝♦❧❛s P❡❧t✐❡r ❈♦♠❜✐♥✐♥❣ ❙✉♣❡r♣♦s✐t✐♦♥ ❛♥❞ ■♥❞✉❝t✐♦♥

■♥tr♦❞✉❝t♦r② ❡①❛♠♣❧❡ ✭✷✮ ❚❤✐s ♣r♦❜❧❡♠ ❝❛♥♥♦t ❜❡ st❛t❡❞ ✐♥ ✜rst✲♦r❞❡r ❧♦❣✐❝ ✭ n ∈ N ✮ ❆♥ ✐♥❞✉❝t✐✈❡ ♣r♦♣❡rt② ♦❢ t❤❡ ❢♦r♠ ∀ n , l ∃ x φ ▼✉st ❝♦♠❜✐♥❡✿ ❙t❛♥❞❛r❞ ❡q✉❛t✐♦♥❛❧ r❡❛s♦♥✐♥❣ ✇✐t❤ ✉♥✐✜❝❛t✐♦♥ t♦✿ ❋✐♥❞ t❤❡ ✈❛❧✉❡ ♦❢ x ✭✇✳r✳t✳ n , l ✮ ✶ ❈❤❡❝❦ t❤❛t ✐t ✐♥❞❡❡❞ ❢✉❧✜❧❧s t❤❡ ❞❡s✐r❡❞ ♣r♦♣❡rt② ✷ ■♥❞✉❝t✐✈❡ r❡❛s♦♥✐♥❣ ♦♥ n ❆❜❞❡❧❦❛❞❡r ❑❡rs❛♥✐ ❛♥❞ ◆✐❝♦❧❛s P❡❧t✐❡r ❈♦♠❜✐♥✐♥❣ ❙✉♣❡r♣♦s✐t✐♦♥ ❛♥❞ ■♥❞✉❝t✐♦♥

■♥tr♦❞✉❝t♦r② ❡①❛♠♣❧❡ ✭✸✮ ❙tr❛✐❣❤t❢♦r✇❛r❞ ❛♣♣r♦❛❝❤✿ ✉s❡ st❛♥❞❛r❞ ♣r♦♦❢ ♣r♦❝❡❞✉r❡s ❢♦r ✜rst✲♦r❞❡r ❧♦❣✐❝ t♦❣❡t❤❡r ✇✐t❤ ❡①♣❧✐❝✐t ✐♥❞✉❝t✐♦♥ s❝❤❡♠❡s ( ψ ( ✵ ) ∧ ∀ n ψ ( n ) ⇒ ψ ( s ( n ))) ⇒ ∀ n ψ ( n ) ❢♦r s♦♠❡ ✏✇❡❧❧✲❝❤♦s❡♥✑ ❢♦r♠✉❧❛ ψ ❖✉r ❛♣♣r♦❛❝❤✿ tr② t♦ ❞✐s❝♦✈❡r ❛✉t♦♠❛t✐❝❛❧❧② s✉❝❤ ✐♥❞✉❝t✐✈❡ ❧❡♠♠❛t❛✱ ❜② ❞❡t❡❝t✐♥❣ ❝②❝❧❡s ✐♥ t❤❡ s❡❛r❝❤ s♣❛❝❡ ❆❜❞❡❧❦❛❞❡r ❑❡rs❛♥✐ ❛♥❞ ◆✐❝♦❧❛s P❡❧t✐❡r ❈♦♠❜✐♥✐♥❣ ❙✉♣❡r♣♦s✐t✐♦♥ ❛♥❞ ■♥❞✉❝t✐♦♥

P❧❛♥ ♦❢ t❤❡ t❛❧❦ ❚❤❡ ❧❛♥❣✉❛❣❡ ❆ ♣r♦♦❢ ♣r♦❝❡❞✉r❡✿ s✉♣❡r♣♦s✐t✐♦♥ ✰ ❧♦♦♣ ❞❡t❡❝t✐♦♥ ❆ ❝②❝❧❡ ❞❡t❡❝t✐♦♥ ❛❧❣♦r✐t❤♠ ❊①♣❡r✐♠❡♥t❛t✐♦♥s ❆❜❞❡❧❦❛❞❡r ❑❡rs❛♥✐ ❛♥❞ ◆✐❝♦❧❛s P❡❧t✐❡r ❈♦♠❜✐♥✐♥❣ ❙✉♣❡r♣♦s✐t✐♦♥ ❛♥❞ ■♥❞✉❝t✐♦♥

❚❤❡ ❧❛♥❣✉❛❣❡ ❈❧❛✉s❛❧ ✭✜rst✲♦r❞❡r✮ ❧♦❣✐❝ ✰ ❛ ✭✉♥✐q✉❡✮ ❛r✐t❤♠❡t✐❝ ♣❛r❛♠❡t❡r n ❚✇♦ s♦rts ι ✭st❛♥❞❛r❞ t❡r♠s✮ ❛♥❞ ω ✭♥❛t✉r❛❧ ♥✉♠❜❡rs✮✱ ✇✐t❤ ✵ : ω, s : ω → ω ❆ s♣❡❝✐❛❧ ❝♦♥st❛♥t s②♠❜♦❧ n ❞❡♥♦t✐♥❣ ❛ ♥❛t✉r❛❧ ♥✉♠❜❡r ❚❡r♠s✱ ✭❡q✉❛t✐♦♥❛❧✮ ❧✐t❡r❛❧s ❛♥❞ ❝❧❛✉s❡s ❛r❡ ❞❡✜♥❡❞ ❛s ✉s✉❛❧ ❞♦ ♥♦t ❝♦♥t❛✐♥ t❤❡ s♣❡❝✐❛❧ s②♠❜♦❧ n n ✲❝❧❛✉s❡s✿ ❝♦♥str❛✐♥❡❞ ❝❧❛✉s❡s ♦❢ t❤❡ ❢♦r♠ [ C | X ] ✇❤❡r❡✿ C ✐s ❛ ❝❧❛✉s❡ X ✐s ♦❢ t❤❡ ❢♦r♠ � k i = ✶ n = t i ✱ ✇❤❡r❡ t ✶ , . . . , t k ✭ k ≥ ✵✮ ❛r❡ t❡r♠s ♦❢ s♦rt ω ❆❜❞❡❧❦❛❞❡r ❑❡rs❛♥✐ ❛♥❞ ◆✐❝♦❧❛s P❡❧t✐❡r ❈♦♠❜✐♥✐♥❣ ❙✉♣❡r♣♦s✐t✐♦♥ ❛♥❞ ■♥❞✉❝t✐♦♥

❙❡♠❛♥t✐❝s ❚❤❡ s♣❡❝✐❛❧ s②♠❜♦❧ n ✐s ✐♥t❡r♣r❡t❡❞ ❛s ❛ t❡r♠ ♦❢ t❤❡ ❢♦r♠ s m ( ✵ ) ✭ m ∈ N ✮ ✵ ❛♥❞ s ❛r❡ ✐♥t❡r♣r❡t❡❞ ❛s ✵ ❛♥❞ s✉❝❝❡ss♦r ❢✉♥❝t✐♦♥ ❚❤❡ ♦t❤❡r s②♠❜♦❧s ❛r❡ ✐♥t❡r♣r❡t❡❞ ❛s ✉s✉❛❧ [ C | � k i = ✶ n = t i ] ❤♦❧❞s ✐♥ I ✐✛ ❢♦r ❡✈❡r② s✉❜st✐t✉t✐♦♥ σ s✉❝❤ t❤❛t I ( n ) = t i σ ✱ C σ ❤♦❧❞s ✐♥ I ❆❜❞❡❧❦❛❞❡r ❑❡rs❛♥✐ ❛♥❞ ◆✐❝♦❧❛s P❡❧t✐❡r ❈♦♠❜✐♥✐♥❣ ❙✉♣❡r♣♦s✐t✐♦♥ ❛♥❞ ■♥❞✉❝t✐♦♥

❚❤❡ ❧❛♥❣✉❛❣❡ ✭✷✮ ❘❡♠❛r❦s✿ ❆ str✐❝t ❡①t❡♥s✐♦♥ ♦❢ ✜rst✲♦r❞❡r ❧♦❣✐❝ ❚❤❡ ❝♦♥st❛♥t n ❞♦❡s ♥♦t ♦❝❝✉r ✐♥ t❤❡ ❝❧❛✉s❡s ❆ ❢♦r♠✉❧❛ ♦❢ t❤❡ ❢♦r♠ f ( n ) = a ♠✉st ❜❡ ✇r✐tt❡♥✿ [ f ( x ) = a | n = x ] ❊①t❡♥s✐♦♥ t♦ ❢♦r♠✉❧æ ✇✐t❤ s❡✈❡r❛❧ ♣❛r❛♠❡t❡rs ❆❜❞❡❧❦❛❞❡r ❑❡rs❛♥✐ ❛♥❞ ◆✐❝♦❧❛s P❡❧t✐❡r ❈♦♠❜✐♥✐♥❣ ❙✉♣❡r♣♦s✐t✐♦♥ ❛♥❞ ■♥❞✉❝t✐♦♥

❆ t❤❡♦r❡t✐❝❛❧ ❧✐♠✐t❛t✐♦♥ ❚❤❡♦r❡♠ ❚❤❡ s❡t ♦❢ s❛t✐s✜❛❜❧❡ s❡ts ♦❢ n ✲❝❧❛✉s❡s ✐s ♥❡✐t❤❡r r❡❝✉rs✐✈❡❧② ❡♥✉♠❡r❛❜❧❡ ✭♦❢ ❝♦✉rs❡ ✦✮ ♥♦r ❝♦✲r❡❝✉rs✐✈❡❧② ❡♥✉♠❡r❛❜❧❡ ❉❡♣❛rt ❢r♦♠✿ ❋✐rst✲♦r❞❡r ❧♦❣✐❝ ✭✉♥s❛t✐s✜❛❜✐❧✐t② ✐s s❡♠✐✲❞❡❝✐❞❛❜❧❡✮ ❘❡✇r✐t❡✲❜❛s❡❞ ✐♥❞✉❝t✐✈❡ t❤❡♦r❡♠ ♣r♦✈✐♥❣ ✭♥♦♥✲♣r♦✈❛❜✐❧✐t② ✐s s❡♠✐✲❞❡❝✐❞❛❜❧❡✮ ❆❜❞❡❧❦❛❞❡r ❑❡rs❛♥✐ ❛♥❞ ◆✐❝♦❧❛s P❡❧t✐❡r ❈♦♠❜✐♥✐♥❣ ❙✉♣❡r♣♦s✐t✐♦♥ ❛♥❞ ■♥❞✉❝t✐♦♥

❚❤❡ ❧❛♥❣✉❛❣❡ ✭✸✮ Pr♦♣♦s✐t✐♦♥ ❊✈❡r② ✭♥♦♥✲t❛✉t♦❧♦❣✐❝❛❧✮ n ✲❝❧❛✉s❡ ✐s ❡q✉✐✈❛❧❡♥t t♦ ❛♥ n ✲❝❧❛✉s❡ ♦❢ t❤❡ ❢♦r♠ [ C | ⊤ ] ♦r [ C | n = t ] Pr♦♦❢✿ � k i = ✶ n = t i ⇔ n = t ✶ ∧ � k i = ✷ t ✶ = t i ✱ t❤✉s k � [ C | n = t i ] ⇔ [ C σ | n = t ✶ σ ] i = ✶ ✇❤❡r❡ σ = ♠❣✉ ( t ✶ , . . . , t k ) ❛♥❞ k � [ C | n = t i ] ⇔ ⊤ i = ✶ ✐❢ t ✶ , . . . , t k ❛r❡ ♥♦t ✉♥✐✜❛❜❧❡ ❆❜❞❡❧❦❛❞❡r ❑❡rs❛♥✐ ❛♥❞ ◆✐❝♦❧❛s P❡❧t✐❡r ❈♦♠❜✐♥✐♥❣ ❙✉♣❡r♣♦s✐t✐♦♥ ❛♥❞ ■♥❞✉❝t✐♦♥

❊①❛♠♣❧❡ [ f ( x , y ) = a | n = s ( z ) ∧ n = x ∧ n = y ] − → [ f ( s ( z ) , s ( z )) = a | n = s ( z )] [ f ( x , y ) = a | n = s ( x ) ∧ n = ✵ ] − → ⊤ ❆❜❞❡❧❦❛❞❡r ❑❡rs❛♥✐ ❛♥❞ ◆✐❝♦❧❛s P❡❧t✐❡r ❈♦♠❜✐♥✐♥❣ ❙✉♣❡r♣♦s✐t✐♦♥ ❛♥❞ ■♥❞✉❝t✐♦♥

❚❤❡ ❧❛♥❣✉❛❣❡ ✭✹✮ ✸ ❦✐♥❞s ♦❢ n ✲❝❧❛✉s❡s✿ ✶ ❙t❛♥❞❛r❞ ✜rst✲♦r❞❡r ❝❧❛✉s❡s✿ ❡①♣r❡ss ✉♥✐✈❡rs❛❧ ♣r♦♣❡rt✐❡s✱ ♥♦t ❞❡♣❡♥❞✐♥❣ ♦♥ t❤❡ ✈❛❧✉❡ ♦❢ n ✷ [ C | n = s k ( ✵ )] ✿ ❡①♣r❡ss❡s ❛ ♣r♦♣❡rt② t❤❛t ❤♦❧❞s ♦♥❧② ✐❢ n ❤❛s s♦♠❡ s♣❡❝✐✜❝ ✈❛❧✉❡ ✭ n = k ✮ ✸ [ C [ x ] | n = s k ( x )] ✿ ❡①♣r❡ss❡s ❛ ♣r♦♣❡rt② C t❤❛t ❤♦❧❞s ❢♦r x = n − k ❆❜❞❡❧❦❛❞❡r ❑❡rs❛♥✐ ❛♥❞ ◆✐❝♦❧❛s P❡❧t✐❡r ❈♦♠❜✐♥✐♥❣ ❙✉♣❡r♣♦s✐t✐♦♥ ❛♥❞ ■♥❞✉❝t✐♦♥