# The Light Switch Problem - Numberphile | Summary and Q&A

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February 16, 2023
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Numberphile
The Light Switch Problem - Numberphile

## TL;DR

After 100 individuals toggle light switches based on specific patterns, it is only the square numbers that remain on, as they have an odd number of factors.

## Key Insights

• #️⃣ The light switches that remain on after the puzzle are the square numbers, which have an odd number of factors.
• 🧑‍🏭 Factors come in pairs, except when there are duplicates, leading to an odd number of factors.
• 🧑‍🏭 The Fundamental Theorem of Arithmetic provides insights into the unique prime factor decomposition of a number.
• #️⃣ The number of factors a number has is crucial in determining whether a light switch is on or off.
• 🥺 Prime numbers play a role in the puzzle, as their factors are only switched twice, leading to the switches being off.
• #️⃣ Squares and numbers with an odd number of factors are associated with switches that remain on.
• 🧑‍🏭 The puzzle showcases the concept of factors, divisors, and the impact of duplication on the number of factors.

## Transcript

I'd like to tell you about a problem that's  very famous in some circles; I've seen it   used as like an interview question for uh  admissions tests, but I've also seen it just   as a puzzle. I would like you to imagine Brady  there are 100 light switches in front of you.  At the moment they're all off. The first  person comes along and turns them ... Read More

### Q: What patterns do the individuals follow when toggling the light switches?

The first person turns on all the switches, while subsequent individuals toggle switches based on factors, turning them on or off.

### Q: How do squares differ from other numbers in terms of factors?

Squares have an odd number of factors due to the duplication of certain factors, whereas other numbers have an even number of factors.

### Q: Can any number have an odd number of factors?

Only square numbers have an odd number of factors, as they possess duplicates among their factors.

### Q: What is the significance of the Fundamental Theorem of Arithmetic in understanding factors?

The theorem states that every number has a unique prime factor decomposition, which can help identify factors and prime factors of a given number.

## Summary & Key Takeaways

• The light switch puzzle involves 100 individuals toggling switches according to specific patterns.

• The switches that remain on at the end are the square numbers, indicating that they have an odd number of factors.

• Factors come in pairs, but squares have additional factors that are duplicates, resulting in an odd number of factors.