# Number of ways for total spin-1

tanaygupta2000
Homework Statement:
Enumerate the number of independent ways in which 2N spin-1/2's can be com-
bined to form a total spin-1. Here, N is an integer.
Relevant Equations:
Addition of spins, s1 + s2 = S
s1 = s2 = 1/2
To the extent I understood this question, we have to enumerate the total no. of ways to get a total of spin S = 1 from 2N number of spin-1/2's.
Now, I think that by spin-1/2's, the question is referring to s1 = s2 = 1/2 (and not something like 3/2, 5/2, ...).
When N = 1, we have 1/2 + 1/2 => No. of ways = 1
When N = 2, we have 1/2 + 1/2 + 1/2 + 1/2 => No. of ways = 3 + 2 + 1 = 6
Likewise,
For N = N, No. of ways should be given by = (2N-1)+(2N-2)+(2N-3)+...+3+2+1
I am sure I'm missing many things here, this question carries 15 marks.
Kindly help !

Homework Helper
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When N = 2, we have 1/2 + 1/2 + 1/2 + 1/2
That does not add up to 1, and I don't see how it would lead to 6 anyway.

tanaygupta2000
Yes sir.
• For two spins => 1/2 + 1/2 = 1 way
• For four spins => (-1/2) + 1/2 + 1/2 + 1/2 = 4 ways since minus sign can be on any of the 4 spins
• For six spins => (-1/2) + (-1/2) + 1/2 + 1/2 + 1/2 + 1/2 = 15 ways of arranging minus signs
• Likewise, for 2N spins, total no. of ways = 2N[C](2N-1) (C --> Combination)
and so on?
Is this method okay as required in the question?

Last edited:
Homework Helper
Gold Member
2022 Award
Yes sir.
• For two spins => 1/2 + 1/2 = 1 way
• For four spins => (-1/2) + 1/2 + 1/2 + 1/2 = 4 ways since minus sign can be on any of the 4 spins
• For six spins => (-1/2) + (-1/2) + 1/2 + 1/2 + 1/2 + 1/2 = 15 ways of arranging minus signs
• Likewise, for 2N spins, total no. of ways = 2N[C](2N-1) (C --> Combination)
and so on?
Is this method okay as required in the question?
Your specific cases look good, but don’t match your general formula (##^{2N}C_{2N-1}##).

tanaygupta2000
Your specific cases look good, but don’t match your general formula (##^{2N}C_{2N-1}##).
Yes sir, sorry I meant ##^{2N}C_{N-1}## number of ways are possible for 2N spins.