# Primes: popular and lonely

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ulam While doodling in class, I made a 10 x 10 grid and filled it with numbers from 1 to 100. The motivations behind 10 x 10 grid was human bias towards the number 10.

Then inspired by Ulam Spiral, I started creating paths (allowing diagonal, horizontal and vertical moves) starting from the smallest number. Following paths emerged:

• 2→ 3 →13 → 23
• 2 → 11
• 7 → 17
• 19 → 29
• 31 → 41
• 37 → 47
• 43 → 53
• 61 → 71
• 73 → 83
• 79 → 89

So, longest path is of length 4 and others are of length 2.

The number 2 is special one here, since it leads to two paths. I will call such primes, with more than one paths, popular primes.

Now, 5, 59, 67 and 97 don’t have any prime number neighbour. I will call such primes, with no neighbour, lonely primes.

I hope to create other $b \times b$ grids filled with 1 to $b^2$ natural numbers written in base $b$. Then will try to identify such lonely and popular primes.

### 8 responses

1. Pingback: Ulam Spiral | Gaurish4Math

• Thanks! This is very accessible exposition of Green-Tao Theorem.

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2. When you do b^2 it wont be prime, ever.

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• Yes, like 10^2 = 100. I just want to preserve this pattern. of 1 to 100 🙂

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• Oh sorry, I see now. I misunderstood!

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