# Off the Number Line: Prime Cut Special

Interviewer:
I get it. Instead of a prime cut of beef, he’s cutting into prime numbers. But what is confusing me is that I see a 6 and a 9, and those aren’t prime. What am I missing?

Cartoonist:
This is fascinating. Check out that number he is cutting into. No matter where that numerical butcher cuts, the right side will always be a prime number. Right now he’s cutting to make 29137, which is prime, but if he moves the knife a digit to the right, 9137 is also prime. If he moves to the left, 629137, he also gets prime. In fact he can cut anywhere along this sequence of digits and he’ll always get a prime cut.

Interviewer:
Wow! That’s awesome. The customer is obscuring the digits on the left. How long does it go?

Cartoonist:
Sequences of digits like this create what are called left truncatable prime numbers, and the longest possible in base 10 is twenty-four digits:

357686312646216567629137

Interviewer:
So this number really is a cut above the rest! Thank you for introducing me to this concept. Looking forward to the next Off the Number Line cartoon!

Matthew Peterson, Ph.D., is Co-founder and Chief Research & Development Officer at the MIND Research Institute. James Huang is Senior Visual Designer at MIND Research Institute.

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