1+2+3+4+5…..= -1/12
Yeah! The fact that the sum of natural numbers till infinity is equal to -1/12 completely blew me.
It's weird and nonsensical to even think that the sum of positive integers till infinity can give a definite value, that too a negative fraction.
Well, this equation was given by Ramanujan and is known as The Ramanujan Summation. Let's look over the proof now.
Let, A = 1-1+1-1+1-1….
The summation of the above series can be 0 or 1, depending upon the number of terms it has, even or odd respectively.
Now subtracting A from 1 we get,
1-A = 1-(1-1+1-1+1….)
1-A = 1-1+1-1….
Eventually, the Right Hand Side equals A.
So, 1-A = A
Or A =1/2
Now let's take another series—
B= 1-2+3-4+5-6……
Therefore,
A-B = (1-1)+(2-1)+(-3+1)+(4-1)….
A-B = 1-2+3–4+5–6…. = B
A=2B
B= 1/4
Now coming to our main interest, let—
C = 1+2+3+4+5….
C-B = (1-1) + {2-(-2)} + (3-3) + {4-(-4)}….
C-B = 4+8+12….
C-B = 4(1+2+3+4…..)
C-B = 4(C)
3C = -B
C = -1/12
Amazing! Isn't it?
But we know this isn't true. The sum of infinite positive integral series must be an infinity. Hence, the result makes no sense.
Then where are we wrong?
We were wrong at assuming the value of 1+2+3….=C. By doing this, we had already assumed it's value to be a well defined where all the algebraic calculations could be done. But this is untrue. How can we assume it's value to be a real quantity when it is infinite?
But this equation has it's use in String Theory and is still a debatable equation of all time!
SS_
Sources:-
The Ramanujan Summation: 1 + 2 + 3 + ⋯ + ∞ = -1/12?




