# Tag Info

Accepted

• 14.5k
Accepted

• 3,326

### Today is a prime!

Here is a year when there definitely will be none Because A smaller example
• 137k

• 2,684

i) ii)
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• 101

### Primeable numbers

Allowing for zeros to be discarded to form primes with fewer digits, the first sequence containing ten or more consecutive non-primeable numbers is: Related sequences: Searching all numbers with six ...
• 15.1k
Accepted

### A Prime Ant's Excursion in the Cartesian Plane

Answer (b) Edit: I was beginning to think that there are only prime solutions, but
• 14.5k

### A Prime Ant's Excursion in the Cartesian Plane

Here's a (messy) picture proof of optimality for @melmackian's solution: Legend: Red lines: first 3 moves (2,3,5 units) Small black circles with coordinates: some lattice points at a too small prime ...
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### A Prime Number equation using all nine digits once

Late, but without brute forcing combinations: Now What about two equals signs? So, we use We note
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### A Prime Number equation using all nine digits once

First off, you will need a minimum of three 2-digit numbers, because none of 1, 4, 6, 8, or 9 is prime, so they will need to be combined with another digit in order to make a prime number. It quickly ...
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### Primeable numbers

Let's try to understand the probability of very large unprimeable numbers. To do so, let's start by looking at how many permutations a number has. If a number has only a single unique digit, it has ...
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Accepted

### Intermingled primes

The 6 different primes are: Deductions (I solved it with regular expressions in a list of all prime numbers from 101-997):
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### Prime to Prime Sequel

Computer confirms that the only other solution is:
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### Number of 1's needed to write all primes up to P

A partial answer with some observations about numerical behavior. You can find the code to compute the numbers in this answer here. First we want to define the ratio $R_N$ as the number of ones in ...
Accepted

### Number of 1's needed to write all primes up to P

The prime-counting function, denoted by $\pi(x)$, counts the number of prime numbers less than or equal to $x$. It is closely approximated by $\frac{x}{\ln{x}}$. If we take $x=10^n$, we know that all ...
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### Primeable numbers

The first such sequence with 10 or more terms is because If we assume that leading zeros are allowed, so that e.g. $203$ is primeable because $23$ (or $023$) is prime, then a similar argument to the ...
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### A Prime Ant's Excursion in the Cartesian Plane

The ant can return home in I'm not sure if points on the axes count as "in the first quadrant", but I think it's reasonable to permit them, as we require the origin to be the start/end ...
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1 vote

### Prime Boggle Game

EDIT: I missed the requirement that primes must be unique. That invalidates this first grid. But the final solution should still be fine. The longest game I can find comprises 39 moves. This is ...
• 26.3k
1 vote

### Prime to Prime: Get all first 25 Prime Numbers using up to 4 Primes

Using 2, 3, 5, 17 2 = 2 3 = 3 5 = 5 7 = 5 + 2 11 = 17 - 3! 13 = 5 × 2 + 3 17 = 5 × 3 + 2 19 = 17 + 2 23 = 17 + 3! 29 = 17 × 2 - 5 31 = 17 × 2 - 3 37 = 17 × 2 + 3 41 = 17 × 3 - 2 × 5 43 = 5! / 2 - ...
• 1,089

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