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Prime power divisibility (Posted on 2024-10-15) Difficulty: 2 of 5
How many prime numbers p make the number 3p2+p+1 + 7p2+p+1 divisible by p?

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We are tasked with finding how many prime numbers <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>p</mi></mrow><annotation encoding="application/x-tex">p</annotation></semantics></math> make the expression <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>3</mn><msup><mi>p</mi><mn>2</mn></msup><mo>+</mo><mi>p</mi><mo>+</mo><mn>1</mn><mo>+</mo><mn>7</mn><msup><mi>p</mi><mn>2</mn></msup><mo>+</mo><mi>p</mi><mo>+</mo><mn>1</mn></mrow><annotation encoding="application/x-tex">3p^2 + p + 1 + 7p^2 + p + 1</annotation></semantics></math> divisible by <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>p</mi></mrow><annotation encoding="application/x-tex">p</annotation></semantics></math>.

First, simplify the given expression:

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><mn>3</mn><msup><mi>p</mi><mn>2</mn></msup><mo>+</mo><mi>p</mi><mo>+</mo><mn>1</mn><mo>+</mo><mn>7</mn><msup><mi>p</mi><mn>2</mn></msup><mo>+</mo><mi>p</mi><mo>+</mo><mn>1</mn><mo>=</mo><mo stretchy="false">(</mo><mn>3</mn><msup><mi>p</mi><mn>2</mn></msup><mo>+</mo><mn>7</mn><msup><mi>p</mi><mn>2</mn></msup><mo stretchy="false">)</mo><mo>+</mo><mo stretchy="false">(</mo><mi>p</mi><mo>+</mo><mi>p</mi><mo stretchy="false">)</mo><mo>+</mo><mo stretchy="false">(</mo><mn>1</mn><mo>+</mo><mn>1</mn><mo stretchy="false">)</mo><mo>=</mo><mn>10</mn><msup><mi>p</mi><mn>2</mn></msup><mo>+</mo><mn>2</mn><mi>p</mi><mo>+</mo><mn>2.</mn></mrow><annotation encoding="application/x-tex">3p^2 + p + 1 + 7p^2 + p + 1 = (3p^2 + 7p^2) + (p + p) + (1 + 1) = 10p^2 + 2p + 2.</annotation></semantics></math>

Now, we want to determine for which prime numbers <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>p</mi></mrow><annotation encoding="application/x-tex">p</annotation></semantics></math>, the expression <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>10</mn><msup><mi>p</mi><mn>2</mn></msup><mo>+</mo><mn>2</mn><mi>p</mi><mo>+</mo><mn>2</mn></mrow><annotation encoding="application/x-tex">10p^2 + 2p + 2</annotation></semantics></math> is divisible by <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>p</mi></mrow><annotation encoding="application/x-tex">p</annotation></semantics></math>. To do this, we need to check the divisibility condition:

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><mn>10</mn><msup><mi>p</mi><mn>2</mn></msup><mo>+</mo><mn>2</mn><mi>p</mi><mo>+</mo><mn>2</mn><mo>≡</mo><mn>0</mn><mspace></mspace><mspace width="1em"></mspace><mo stretchy="false">(</mo><mrow><mi mathvariant="normal">m</mi><mi mathvariant="normal">o</mi><mi mathvariant="normal">d</mi></mrow><mspace width="0.3333em"></mspace><mi>p</mi><mo stretchy="false">)</mo><mi mathvariant="normal">.</mi></mrow><annotation encoding="application/x-tex">10p^2 + 2p + 2 \equiv 0 \pmod{p}.</annotation></semantics></math>

By reducing the expression modulo <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>p</mi></mrow><annotation encoding="application/x-tex">p</annotation></semantics></math>, we get:

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><mn>10</mn><msup><mi>p</mi><mn>2</mn></msup><mo>+</mo><mn>2</mn><mi>p</mi><mo>+</mo><mn>2</mn><mo>≡</mo><mn>0</mn><mspace></mspace><mspace width="1em"></mspace><mo stretchy="false">(</mo><mrow><mi mathvariant="normal">m</mi><mi mathvariant="normal">o</mi><mi mathvariant="normal">d</mi></mrow><mspace width="0.3333em"></mspace><mi>p</mi><mo stretchy="false">)</mo><mi mathvariant="normal">.</mi></mrow><annotation encoding="application/x-tex">10p^2 + 2p + 2 \equiv 0 \pmod{p}.</annotation></semantics></math>

Since <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msup><mi>p</mi><mn>2</mn></msup><mo>≡</mo><mn>0</mn><mspace></mspace><mspace width="0.4444em"></mspace><mo stretchy="false">(</mo><mrow><mi mathvariant="normal">m</mi><mi mathvariant="normal">o</mi><mi mathvariant="normal">d</mi></mrow><mspace width="0.3333em"></mspace><mi>p</mi><mo stretchy="false">)</mo></mrow><annotation encoding="application/x-tex">p^2 \equiv 0 \pmod{p}</annotation></semantics></math> and <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>p</mi><mo>≡</mo><mn>0</mn><mspace></mspace><mspace width="0.4444em"></mspace><mo stretchy="false">(</mo><mrow><mi mathvariant="normal">m</mi><mi mathvariant="normal">o</mi><mi mathvariant="normal">d</mi></mrow><mspace width="0.3333em"></mspace><mi>p</mi><mo stretchy="false">)</mo></mrow><annotation encoding="application/x-tex">p \equiv 0 \pmod{p}</annotation></semantics></math>, the expression simplifies to:

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><mn>10</mn><mo stretchy="false">(</mo><mn>0</mn><mo stretchy="false">)</mo><mo>+</mo><mn>2</mn><mo stretchy="false">(</mo><mn>0</mn><mo stretchy="false">)</mo><mo>+</mo><mn>2</mn><mo>≡</mo><mn>2</mn><mspace></mspace><mspace width="1em"></mspace><mo stretchy="false">(</mo><mrow><mi mathvariant="normal">m</mi><mi mathvariant="normal">o</mi><mi mathvariant="normal">d</mi></mrow><mspace width="0.3333em"></mspace><mi>p</mi><mo stretchy="false">)</mo><mi mathvariant="normal">.</mi></mrow><annotation encoding="application/x-tex">10(0) + 2(0) + 2 \equiv 2 \pmod{p}.</annotation></semantics></math>

Therefore, the condition becomes:

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><mn>2</mn><mo>≡</mo><mn>0</mn><mspace></mspace><mspace width="1em"></mspace><mo stretchy="false">(</mo><mrow><mi mathvariant="normal">m</mi><mi mathvariant="normal">o</mi><mi mathvariant="normal">d</mi></mrow><mspace width="0.3333em"></mspace><mi>p</mi><mo stretchy="false">)</mo><mi mathvariant="normal">.</mi></mrow><annotation encoding="application/x-tex">2 \equiv 0 \pmod{p}.</annotation></semantics></math>

This implies that <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>p</mi></mrow><annotation encoding="application/x-tex">p</annotation></semantics></math> must divide 2. The only prime number that divides 2 is <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>p</mi><mo>=</mo><mn>2</mn></mrow><annotation encoding="application/x-tex">p = 2</annotation></semantics></math>.

Thus, the only prime number that satisfies the divisibility condition is <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>p</mi><mo>=</mo><mn>2</mn></mrow><annotation encoding="application/x-tex">p = 2</annotation></semantics></math>.

Answer: There is 1 prime number, which is <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>p</mi><mo>=</mo><mn>2</mn></mrow><annotation encoding="application/x-tex">p = 2</annotation></semantics></math>

Sprunki Retake


  Posted by BevisJason on 2025-02-14 21:27:52
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