# a spectacular solution to the Basel problem (sum of 1/n^2 via a complex integral) | Summary and Q&A

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August 23, 2018
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blackpenredpen
a spectacular solution to the Basel problem (sum of 1/n^2 via a complex integral)

## TL;DR

The video discusses complex integrals and uses them to solve the Basel Problem, resulting in the famous result of π^2/6.

## Key Insights

• 💁 Complex integrals can be used to solve real integrals by converting them into a complex form.
• 🤝 Series expansion is a useful technique to solve complex integrals, especially when dealing with logarithmic functions.
• 🛀 The famous result of the Basel Problem, π^2/6, can be derived using the complex integral technique shown in the video.
• 0️⃣ The imaginary part of a real integral must be zero for the integral to have a real value.

## Transcript

this integral it's super amazing in fact not the  real version of it even though everything's real   but rather we'll talk about the complex version  of this and that's the one that's going to keep   us very very nice result (solution to the Basel  Problem) and you guess I kinda liked it so much   I didn't come with the following though I want  to ... Read More

### Q: What is the complex definition of cosine used in the video?

The complex definition of cosine used in the video is e^(ix) + e^(-ix)/2.

### Q: How is the complex integral separated into two parts?

The presenter factors out e^(ix) from the integral, resulting in the sum of ln(e^(ix)) and ln(e^(-ix)).

### Q: Why does the presenter choose to solve the second integral over the first one?

The presenter chooses to solve the second integral because the ln and e terms cancel each other out, making the integration simpler.

### Q: How is series expansion used to solve the complex integral?

The presenter applies the series expansion for ln(1 + x) to the e^(-2ix) term in the integral. This expansion allows the integration to be expressed as a sum of series terms.

## Summary & Key Takeaways

• The content introduces the concept of complex integrals, specifically using the complex version of the integral from 0 to π/2 of ln(2*cos(x)).

• The presenter shows the steps to convert the real integral into a complex one using the complex definition of cosine.

• The video demonstrates the integration of the complex integral and separates it into two parts.

• The presenter applies series expansion to solve the complex integral and derives the result of π^2/6 for the Basel Problem.