What are some examples of obsolete letters from the Greek alphabet

Greek alphabet - Wikimedia Commons

Greek Numerals -- from Wolfram MathWorld

What letters are missing from the Greek alphabet? - Quora

What letters are missing from the Greek alphabet? - Quora

What ancient Greek letters are still used today? - Quora

What ancient Greek letters are still used today? - Quora

Numerals

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Why do so many logos replace the letter A with the Greek letter lambda? - Quora