Computabilità 32
Cardinalità di insiemi di funzioni
• Due domande interessanti:
– Quante funzioni f: ℕ → ℕ ci sono?
– Quali sono le funzioni f y : ℕ → ℕ?
• Cominciamo con “quante”:
{f: ℕ → ℕ}⊇{f: ℕ → {0,1} } ⇒
|{f: ℕ → ℕ }| ≥ |{f: ℕ → {0,1} }| = |℘(ℕ)| = 2
= cardinalità dell’insieme ℕ dei numeri naturali
– si legge “alef con zero”
• 2 = cardinalità dell’insieme ℝ dei numeri reali
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Computabilità 33
Problemi e soluzioni
• Insieme di problemi: |{f: ℕ → ℕ }| ≥ | {f: ℕ → {0,1} }|
= |℘(ℕ)| = 2
• L’insieme di funzioni calcolate da TM {f y : ℕ → ℕ} è per definizione numerabile
– NB: E: {M y } ↔ ℕ induce E ^ : ℕ → {f y } non uno a uno (in molti casi f y = f z , con z ≠ y) ma basta per poter stabilire che
|{f y : ℕ → ℕ}| = < 2 = |{f: ℕ → ℕ }|
→ la “maggior parte” dei problemi non può essere risolta algoritmicamente!
– Ci sono molti più problemi che programmi!
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