Limit inferior of difference of consecutive primes was 70 million

A × B × C × D × E × F = 7 × 1 0 7 \large A \times B \times C \times D \times E \times F = 7 \times 10^7

Determine the number of ordered solutions for integers A , B , C , D , E , F A,B,C,D,E,F such that they satisfy the equation above.

Inspiration .


The answer is 120434688.

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1 solution

Pi Han Goh
Apr 30, 2015

The factorization of 7 × 1 0 7 7 \times 10^7 is simply 2 7 × 5 7 × 7 2^7 \times 5^7 \times 7 .

Suppose for now we restrict the search for positive integer solutions only.

For non-negative integers x i , y i , z i x_i, y_i, z_i , let A = 2 x 1 5 y 1 7 z 1 , B = 2 x 2 5 y 2 7 z 2 , , F = 2 x 6 5 y 6 7 z 6 A = 2^{x_1} \cdot 5^{y_1} \cdot 7^{z_1}, B = 2^{x_2} \cdot 5^{y_2} \cdot 7^{z_2} , \ldots, F = 2^{x_6} \cdot 5^{y_6} \cdot 7^{z_6} .

Then their product is 2 7 × 5 7 × 7 1 = 2 j = 1 6 x j × 5 j = 1 6 x j × 7 j = 1 6 x j 2^7 \times 5^7 \times 7^1 = 2^{\sum_{j=1}^6 x_j} \times 5^{\sum_{j=1}^6 x_j} \times 7^{\sum_{j=1}^6 x_j} .

j = 1 6 x j = 7 , j = 1 6 y j = 7 , j = 1 6 z j = 1 \Rightarrow \displaystyle \sum_{j=1}^6 x_j = 7, \sum_{j=1}^6 y_j = 7, \sum_{j=1}^6 z_j = 1 .

By Stars and Bars, we have ( 7 + 6 1 6 1 ) = ( 12 5 ) {7+6 - 1 \choose 6-1} = {12 \choose 5} ordered solution for < x i > <x_i> and < y i > <y_i> , and ( 1 + 6 1 6 1 ) = 6 {1+6 - 1 \choose 6-1} = 6 ordered solution for < z i > <z_i> .

Multiplying them together gives a total of ( 12 5 ) 2 × 6 = 3763584 {12 \choose 5} ^2 \times 6 = 3763584 ordered solution for positive integers A A to F F .

If we drop the constraint for positive integers, then we could have:

Case 1 : 6 Positive integers

Case 2 : 4 positive integers, 2 negative integers

Case 3 : 2 positive integers, 4 negative integers

Case 4 : 6 Negative integers

Combining all these cases together yields a total of

3763584 × [ ( 6 0 ) + ( 6 2 ) + ( 6 4 ) + ( 6 6 ) ] = 120434688 3763584 \times \left [ {6 \choose 0} + {6 \choose 2} + {6 \choose 4} + {6 \choose 6} \right ] = \boxed{120434688} ordered solutions.

After you get to 3763584, you can also finish as follows: Choose the signs of A A , B B , \dots , E E arbitrarily. Then the sign of F F is uniquely determined, so you multiply 3763584 by 2 5 2^5 .

Jon Haussmann - 6 years, 1 month ago

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OH NICE! Never thought of "Forcing F F to be uniquely determined".

Pi Han Goh - 6 years, 1 month ago

By this , we can prove that , ( n 0 ) + ( n 2 ) + ( n 4 ) + . . . . + ( n n ) ( w h e n n = e v e n ) \left( \begin{matrix} n \\ 0 \end{matrix} \right) +\left( \begin{matrix} n \\ 2 \end{matrix} \right) +\left( \begin{matrix} n \\ 4 \end{matrix} \right) +....+\left( \begin{matrix} n \\ n \end{matrix} \right) \quad (when\quad n=even) is equal to ( n 0 ) + ( n 2 ) + ( n 4 ) + . . . . + ( n n 1 ) ( w h e n n = o d d ) \left( \begin{matrix} n \\ 0 \end{matrix} \right) +\left( \begin{matrix} n \\ 2 \end{matrix} \right) +\left( \begin{matrix} n \\ 4 \end{matrix} \right) +....+\left( \begin{matrix} n \\ n-1 \end{matrix} \right) \quad (when\quad n=odd) is equal to 2 n 1 { 2 }^{ n-1 }

Vighnesh Raut - 6 years, 1 month ago

Very nice.Even I did the same way

Indraneel Mukhopadhyaya - 5 years, 4 months ago

did the same way...

Vighnesh Raut - 6 years, 1 month ago

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Even i was thinking of the exact same way but i was a bit scared,so:P:P:P

Adarsh Kumar - 6 years, 1 month ago

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I was scare too while submitting the answer but it was correct.... :)

Vighnesh Raut - 6 years, 1 month ago

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@Vighnesh Raut nice!great job!

Adarsh Kumar - 6 years, 1 month ago

Did you mean 6 negative integers for case 4?

Baby Googa - 6 years, 1 month ago

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HAHA, THANKS! FIXED \

Pi Han Goh - 6 years, 1 month ago

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