The magnificence of Infinity!

What is the value of the Cesaro's sum of 1 1 + 1 1 + 1 1 + . . . 1 - 1 + 1 - 1 + 1 - 1 + ... ? If you think that the value of the sum is infinite, enter 2020 -2020 .


The answer is 0.500.

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1 solution

Goh Choon Aik
Aug 3, 2019

Let the equation equate to a variable S S .

1 1 + 1 1 + 1 1 + . . . = S 1 - 1 + 1 - 1 + 1 - 1 + ... = S

From here, we can bracket the equations differently:

( 1 1 ) + ( 1 1 ) + ( 1 1 ) + . . . = S (1 - 1) + (1 - 1) + (1 - 1) + ... = S

1 + ( 1 + 1 ) + ( 1 + 1 ) + ( 1 + 1 ) + . . . = S 1 + (-1 + 1) + (-1 + 1) + (-1 + 1) + ... = S

We can then sum the equations, solving every sub-equation within the brackets:

1 + 0 + 0 + 0 + 0 + 0 + 0 + . . . = 2 S 1 + 0 + 0 + 0 + 0 + 0 + 0 + ... = 2S

S = 1 2 S = \frac{1}{2}

This answer, however, may be wrong as by bracketing the equations differently, we can derive 0 = 1 0 = 1 . I remember reading somewhere you take partial sums of the equation, then average between the answers, which leads to the answer being 1 2 \frac{1}{2} due to the partial equations not converge on a single value but rather jumping between 0 and 1 constantly.

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