Home » Featured » Solving Ramanujan’s Puzzling Problem

Solving Ramanujan’s Puzzling Problem

Consider a sequence of functions as follows:-

f_1 (x) = \sqrt {1+\sqrt {x} }
f_2 (x) = \sqrt{1+ \sqrt {1+2\sqrt {x} } }

f_3 (x) = \sqrt {1+ \sqrt {1+2 \sqrt {1+3 \sqrt {x} } } }

……and so on to

f_n (x) = \sqrt {1+\sqrt{1+2 \sqrt {1+3 \sqrt {\ldots \sqrt {1+n \sqrt {x} } } } } }

Evaluate this function as n tends to infinity.

Or logically:

Find

\displaystyle{\lim_{n \to \infty}} f_n (x) .

Solution

Ramanujan discovered

x+n+a= sqrt{ax + (n+a)^2 +xsqrt{a(x+n)+(n+a)^2 +(x+n)sqrt{...}}}

which gives the special cases

x+1 = sqrt{1+xsqrt{1 + (x+1)sqrt{1 + (x+2)sqrt{1 + (x+2)sqrt{...}}}}}

for x=2, n=1 and a=o

3 = sqrt{1+2sqrt{1+3 sqrt{1+ 4sqrt{1+cdots}}}}

Proof of Ramanujan’s nested radicals equation(x+n+a)^2 = (ax + (n+a)^2+ x(x+2n+a)

(x+n+a) =sqrt{ (ax + (n+a)^2+ x(x+2n+a)}

(x+2n+a) = ((x+n)+n+a)

(x+2n+a) = sqrt{(a(x+n) + (n+a)^2+ (x+n)((x+n)+2n+a)}

(x+2n+a) =sqrt{ (a(x+n) + (n+a)^2+ (x+n)((x+3n+a)}

Then keep replacing the the last part term by writing (alpha x+beta n+a) in the form of (((x+(beta-1)n)+n+a) to give

x+n+a= sqrt{ax + (n+a)^2 +xsqrt{a(x+n)+(n+a)^2 +(x+n)sqrt{...}}}

Which is another form of our problem and is referred as Ramanujan’s Nested Radical. Comparing these two expressions & assuming

=X , we can write the problem as:

\displaystyle {\lim_{n \to \infty}} f_n (x)

= \sqrt {1+X}

= \sqrt {1+3}

=\sqrt {4}

=2

-

Thank you for your precious comments and suggestions.

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11 Comments

  1. Carl Andreas says:

    I’m not entirely sure, but using C++ (with n = 1,000,000) I numerically evaluated it to the function f(x) = 2. But as I said, not quite sure!

  2. You’re right..! After using google, I got this Link , which was also saying the same. But I wasn’t satisfied.

  3. Dave Radcliffe says:

    There are two slightly different versions of this nested radical, so you need to be careful.

    The version posed by Ramanujan was
    sqrt(1 + 2*sqrt(1 + 3*sqrt(1 + 4*sqrt(1 + … = 3

    Your version is almost the same:
    sqrt(1 + 1*sqrt(1 + 2*sqrt(1 + 3*sqrt(1 + … = sqrt(1 + 3) = 2.

  4. Tagus from Reddit says:

    Hi there,

    Just a small typo — I think you meant to write the limit as n tends to infinity. On all of the limits you wrote in that article, you unfortunately said that x goes to infinity.

    x_x

  5. wow – wouldnt have a clue where to start!

  6. Udit Agarwal says:

    I think you might be one of the best bloggers in India today. We are having a TEDx conference, and it would be great to have you as a Speaker. I am sure you can come up with a very interesting talk. Let me know however I can contact you.

  7. karthik says:

    Ramanujan always the best

  8. I really love the version that starts 3= because it smells like a magic number but really implicates the architecture of the number system we use.

  9. narendra hegade says:

    really nice.

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