self doubt

Question

• if we reduce an unknown problem L to a known undecidable problem then L becomes undecidable. is right or wrong??

so it should be known undecidable or only undecidable??

then what about below problem??     this is copied question

If L₁ reduces to L₂ and L₂ is undecidable, then L₁ is undecidable.

Comments

When L1 is reduced to L2,

1) If L1 is Undecidable then L2 is surely Undecidable.

2) If L2 is Decidable then L1 is surely Decidable.

But in your statement, L1 is reduced to L2, If L2 is undecidable then we can't say anything about L1

So the statement is wrong.

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Q Ashwin Kulkarni 

read my topmost statement then answer

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Yes That's what I am saying, if L is unknown and we reduced it to a known undecidable, then we can't surely say L is undecidable.

To prove that L is undecidable you've to again reduce that known undecidable to L.

Answer 1

If your aim is to prove that L1 is undecidable, then we should reduce the known undecidable problem L2 into L1. 

you can refer  here and here