Similar to Ramanujan's Step sum

Algebra Level 5

A = 1 + 1 1 + e π 1 + e 2 π 1 + e 3 π 1 + A=1+\dfrac{1}{1+\dfrac{e^{-\pi}}{1+\dfrac{e^{-2\pi}}{1+\dfrac{e^{-3\pi}}{1+\cdots}}}}

Find A A .

Note: Don't use any programming or software such as Wolfram Alpha.


The answer is 1.9586.

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3 solutions

Mark Hennings
Oct 9, 2019

We have A = 1 + e 1 5 π R ( e π ) A \; = \; 1 + e^{\frac15\pi}R(e^{-\pi}) where R ( q ) R(q) is the Rogers-Ramanujan continued fraction. It is a remarkable fact that R ( e π ) = 1 8 ( 3 + 5 ) ( 5 4 1 ) ( 10 + 2 5 ( 3 + 5 4 ) ( 5 4 1 ) ) R(e^{-\pi}) \; = \; \tfrac18(3 + \sqrt{5})(\sqrt[4]{5}-1)\left(\sqrt{10+2\sqrt{5}} - (3 + \sqrt[4]{5})(\sqrt[4]{5}-1)\right) and so we have an exact expression for A A . To 10 10 significant figures, we have A = 1.958650182 A = \boxed{1.958650182} .

How did they prove this?

Alapan Das - 1 year, 8 months ago

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The Mathworld page for the R-R CF refers to two papers, one in 2001 and the other in 2004.

Mark Hennings - 1 year, 8 months ago
Alapan Das
Oct 9, 2019

It can be proved that the function

ρ ( q ) \rho(q) = 1 + 1 1 + q 1 + q 2 1 + q 3 1 + . . . . . =1+\frac{1}{1+\frac{q}{1+\frac{q^2}{1+\frac{q^3}{1+.....}}}} is continuous ,and of infinite class over the positive real numbers.( In complex field it has singularities.

So, ρ ( q ) = ρ ( 0 ) + q ρ ( 0 ) + q 2 2 ! ρ ( 0 ) + q 3 3 ! ρ ( 0 ) + . . . . \rho(q)=\rho(0)+q\rho'(0)+\frac{q^2}{2!}\rho''(0)+\frac{q^3}{3!}\rho'''(0)+....

We get, ρ ( 0 ) = 2 , ρ ( 0 ) = 1 , ρ ( 0 ) = 2 , ρ ( 0 ) = 4 , . . . . \rho(0)=2,\rho'(0)=-1,\rho''(0)=2,\rho'''(0)=-4, ....

So, ρ ( q ) = 2 q + q 2 q 4 + q 5 q 6 + q 7 q 9 + q 10 q 11 + q 12 q 14 . . . . \rho(q)=2-q+q^2-q^4+q^5-q^6+q^7-q^9+q^{10}-q^{11}+q^{12}-q^{14}.... .

Here q 3 , q 8 , q 13 , q 23 , . . . q^3,q^8,q^{13},q^{23},... terms are invalid. But what is the pattern? Yes. It is similar to Fibonacci sequence. See, the missing powers are 3 , 8 , 13 , 23... 3,8,13,23... and ( 3 + 8 ) + 2 = 13 ; ( 8 + 13 ) + 2 = 23.... (3+8)+2=13;(8+13)+2=23.... So, missing numbers will follow the Fibonacci sequence but 2 '2' ahead.

ρ ( e π ) ( 2 e π + e 2 π e 4 π + q 5 π ) 1.9586 \rho(e^{-\pi})≈(2-e^{-\pi}+e^{-2\pi}-e^{-4\pi}+q^{-5\pi})≈1.9586 .

Hana Wehbi
Oct 13, 2019

I know we need to solve it manually but is there any Python program to solve this, please share.

I have done it with an Excel spreadsheet, if you are interested.

Chew-Seong Cheong - 1 year, 8 months ago

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Thank you. Everything works. I am recently learning python and interested in learning other languages too so l thought there could be a certain program. Thank you always.

Hana Wehbi - 1 year, 8 months ago

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