@Kapil sir please check if my understanding is right

To say language decidable / undecidable we should be able to answer for **ALL **instance of problem

Here instance of problem is given - TM , {Set of all even numbers } , Ques TM accepts all even numbers or not ?

If we use dovetailing - Means run step 1 for String 1 on TM1 then String2 of input2 on TM1 ...........

But here Our UTM will be doing checking on TM1 only forever as set of Strings (even numbers here) is infinite

**So even for YES instance of ****problem**** we have to wait for infinite time **

Hence Non RE and UD

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In https://gateoverflow.in/1994/gate2014-2-35 this problem

Instance of prob - given TM , Set of all Strings of length 2014 , Wherther TM accepts any of them or not ?

If we use dovetailing , Means run step 1 for String 1 on TM1 then String2 of input2 on TM1 .

two cases may happen -- > 1) TM1 halts on some input and we take new TM2 to check on UTM

2) TM1 may go in infinite loop ...

**But for ****atleast**** YES instance of ****problem**** we have to wait for finite time here**

Hence RE (as we can't say about no instance) but not REC