Let's analyze the given sequence.
a_(n+1)={ ll 1/2a_n, & ifa_nis even
3a_n+1, & ifa_nis odd .
We are asked to write terms of that sequence for a_1=34 and for a_1=25.
a_1=34
Let a_1=34. Since a_1=34 is even, the second term is a_2= 12a_1= 12(34)=17. Since a_2=17 is odd, the third term is a_3=3a_2+1=3(17)+1=52. We will continue to do the same until we come across a pattern.
n
a_(n-1)
Is a_(n-1) even?
a_n
1
-
-
a_1=34
2
a_1=34
Yes âś“
a_2=1/2(34)=17
3
a_2=17
No *
a_3=3(17)+1=52
4
a_3=52
Yes âś“
a_4=1/2(52)=26
5
a_4=26
Yes âś“
a_5=1/2(26)=13
6
a_5=13
No *
a_6=3(13)+1=40
7
a_6=40
Yes âś“
a_7=1/2(40)=20
8
a_7=20
Yes âś“
a_8=1/2(20)=10
9
a_8=10
Yes âś“
a_9=1/2(10)=5
10
a_9=5
No *
a_(10)=3(5)+1=16
11
a_(10)=16
Yes âś“
a_(11)=1/2(16)=8
12
a_(11)=8
Yes âś“
a_(12)=1/2(8)=4
13
a_(12)=4
Yes âś“
a_(13)=1/2(4)=2
14
a_(13)=2
Yes âś“
a_(14)=1/2(2)=1
15
a_(14)=1
No *
a_(15)=3(1)+1=4
16
a_(15)=4
Yes âś“
a_(16)=1/2(4)=2
17
a_(16)=2
Yes âś“
a_(17)=1/2(2)=1
We got the term 1 again. This tells us that the sequence 4, 2, 1 will repeat indefinitely.
a_1=25
We will continue to do the same method as the first sequence.
n
a_(n-1)
Is a_(n-1) even?
a_n
1
-
-
a_1=25
2
a_1=25
No *
a_2=3(25)+1=76
3
a_2=76
Yes âś“
a_3=1/2(76)=38
4
a_3=38
Yes âś“
a_4=1/2(38)=19
5
a_4=19
No *
a_5=3(19)+1=58
6
a_5=58
Yes âś“
a_6=1/2(58)=29
7
a_6=29
No *
a_7=3(29)+1=88
8
a_7=88
Yes âś“
a_8=1/2(88)=44
9
a_8=44
Yes âś“
a_9=1/2(44)=22
10
a_9=22
Yes âś“
a_(10)=1/2(22)=11
11
a_(10)=11
No *
a_(11)=3(11)+1=34
Notice that we get a_(11)= 34. Therefore, after this term we will get the same sequence as in the first sequence, where a_1= 34.
Conclusion
Notice that that each time an even number is reached, it is divided until it reaches an odd number. Then, it grows again, but it appears that at some point we reach the number 4, from which the sequence has the subsequence 4, 2, 1, that repeats indefinitely.
Extra
Collatz Conjecture
The Collatz conjecture is a mathematical conjecture related to the sequence in this exercise. The conjecture is that no matter what value of a_1 we start with, the given sequence will always reach the subsequence 4, 2, 1, which repeats indefinitely.
Paul Erdos, a famous mathematician, said about the Collatz conjecture: Mathematicians may not be ready for such problems.
Jeffrey Lagarias, another famous mathematician, said: This is an extraordinarily difficult problem, completely out of reach of present day mathematics.
Even today, mathematicians do not know if the Collatz conjecture is true or false.
