# Primes in the form $n^{k}+n-1$

Lately I've been (very casually) toying with primes in the form $$n^k+n-1$$, as a very far-reaching generalization of Fermat primes. (you get a Fermat prime when you set $$k=2^m$$ and $$n=2$$). I have little training in number theory, so mainly I've been just typing stuff into Wolfram and searching for patterns.

One thing I started doing was looking for the smallest $$k$$ such that $$n^k+n-1$$ is prime for any particular $$n$$. I've been able to find solutions for $$n \leq 106$$. Obviously $$k=1$$ and $$k=2$$ do a lot of heavy lifting, but there are some remarkable numbers here. The one that stunned me the most upon finding it was $$32^{108}+31 \approx 3.6 \times 10^{162}$$, but there's an even bigger smallest solution: $$80^{194}+79 \approx 1.58 \times 10^{369}$$.

For $$n=107$$, there are no solutions for $$k \leq 495$$, which is about as far as I can reliably check. Obviously any $$k$$ that works must be divisible by $$4$$.

So, three things I wonder about are:

1. Is any solution for $$n=107$$ known? Does it exist for sure?
2. In general, has it been shown that there is a solution for any $$n$$?
3. For any $$k$$ not in the form $$6m+5$$, is there $$n$$ such that $$k$$ is the smallest solution?

EDIT:

Let $$f(n)$$ be any polynomial $$\mathbb N \rightarrow \mathbb N$$. Let $$\text{GCD}[f]$$ denote the greatest common divisor of $$\{f(n): n \in \mathbb N\}$$. There is this long-standing conjecture by Bouniakowsky that $$f$$ is irreducible iff there are infinitely many numbers $$n$$ such that $$\frac{f(n)}{\text{GCD}[f]}$$ is prime.

If we suppose this is true, then the following statement implies a positive answer to question 3:

For any $$k: k \not \equiv 5 \pmod 6$$ there are natural numbers $$\alpha,\beta$$ such that $$\text{GCD}[(\alpha n+\beta)^k+(\alpha n+\beta)-1]=1$$, but $$\text{GCD}[(\alpha n + \beta)^m+(\alpha n + \beta)-1] \neq 1$$ for any $$m.

Example: for $$k=3$$, one can let $$\alpha = 15, \beta = 2$$.

To my untrained eye, this seems approachable.

• Unsurprizingly, turns out the sequence is known in OEIS as A076845; but the list ends at 100 Dec 28, 2020 at 22:38
• For $n=107$ and $k=1400$ , we get a prime number (I have not yet proven the primality, but a Miller rabin test with $40$ bases was successful). I am pretty sure that $2$ and $3$ are open questions without a hope to answer them. Dec 29, 2020 at 9:00
• @MichałZapała. For $1$ to $100$ it is almost immediate. After, it takes a lot of time. Dec 29, 2020 at 9:09
• A few more $\{2,44,2,14,3,1,1400,6,3,4,6,1,1\}$ Dec 29, 2020 at 9:12
• @Peter, $n^{6m+5}+n-1$ is divisible by $n^2-n+1$. Dec 29, 2020 at 9:39

Not a 'real' answer, but it was too big for a comment. I think that you're looking for a solution without using a calculator or PC but maybe this gives some insight. I did only a quick search with the following bounds: $$1\le\text{n}\le10^3$$ and $$1\le\text{k}\le10$$.

I wrote and ran some Mathematica-code:

In[1]:=Clear["Global*"];
\[Alpha] = 10^3;
\[Beta] = 10^1;
ParallelTable[
If[PrimeQ[n^k + n - 1], {n, k}, Nothing], {n, 1, \[Alpha]}, {k,
1, \[Beta]}] //. {} -> Nothing


Running the code gives:

Out[1]={{{2, 1}, {2, 2}, {2, 4}, {2, 8}}, {{3, 1}, {3, 2}, {3, 3}, {3,
4}, {3, 8}, {3, 10}}, {{4, 1}, {4, 2}, {4, 3}, {4, 6}, {4, 8}, {4,
9}}, {{5, 2}, {5, 6}, {5, 10}}, {{6, 1}, {6, 2}, {6, 4}, {6,
7}, {6, 10}}, {{7, 1}, {7, 3}}, {{8, 2}, {8, 6}, {8, 10}}, {{9,
1}, {9, 2}, {9, 4}, {9, 7}, {9, 10}}, {{10, 1}, {10, 2}, {10,
3}, {10, 4}, {10, 9}}, {{11, 2}, {11, 8}}, {{12, 1}, {12,
4}}, {{13, 2}, {13, 4}, {13, 10}}, {{15, 1}, {15, 2}, {15, 3}, {15,
9}}, {{16, 1}, {16, 2}, {16, 3}, {16, 4}, {16, 8}, {16,
10}}, {{17, 4}, {17, 8}}, {{18, 3}, {18, 6}}, {{19, 1}, {19,
2}}, {{20, 2}, {20, 4}}, {{21, 1}, {21, 2}, {21, 3}}, {{22,
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10}}, {{25, 3}, {25, 8}}, {{26, 2}, {26, 4}, {26, 10}}, {{27,
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1}, {304, 4}}, {{305, 2}, {305, 6}}, {{306, 2}}, {{307, 1}, {307,
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3}, {310, 8}}, {{312, 8}}, {{313, 3}}, {{314, 2}}, {{316, 1}, {316,
3}}, {{318, 4}}, {{319, 2}, {319, 3}}, {{321, 1}, {321,
4}}, {{322, 1}, {322, 4}}, {{323, 2}, {323, 6}}, {{324, 1}, {324,
10}}, {{326, 8}}, {{327, 1}, {327, 7}, {327, 8}}, {{328,
6}}, {{329, 4}}, {{330, 1}, {330, 2}, {330, 4}, {330, 9}}, {{331,
1}, {331, 2}, {331, 6}}, {{334, 3}, {334, 4}}, {{335, 2}}, {{336,
10}}, {{337, 1}}, {{339, 1}, {339, 2}, {339, 10}}, {{342,
1}}, {{343, 2}, {343, 3}}, {{344, 4}}, {{346, 1}, {346, 9}}, {{348,
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1}, {351, 2}, {351, 4}, {351, 10}}, {{352, 3}, {352, 7}}, {{353,
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8}, {909, 9}}, {{910, 3}}, {{912, 1}}, {{913, 4}, {913, 7}}, {{915,
2}}, {{916, 1}, {916, 8}}, {{917, 8}}, {{918, 6}}, {{919,
8}, {919, 9}}, {{921, 2}, {921, 8}}, {{922, 4}}, {{923, 2}}, {{924,
1}, {924, 4}, {924, 7}}, {{925, 2}, {925, 4}}, {{926, 6}}, {{928,
6}}, {{930, 2}, {930, 9}}, {{931, 1}, {931, 6}}, {{934, 1}}, {{936,
1}}, {{937, 1}, {937, 7}}, {{938, 4}, {938, 10}}, {{939, 1}, {939,
2}, {939, 8}}, {{940, 1}, {940, 3}, {940, 9}, {940, 10}}, {{941,
2}}, {{943, 3}}, {{944, 2}}, {{945, 1}, {945, 9}}, {{946, 2}, {946,
4}}, {{948, 3}, {948, 4}}, {{950, 2}, {950, 6}, {950, 8}}, {{951,
1}, {951, 4}, {951, 8}}, {{952, 4}}, {{953, 4}}, {{954, 1}}, {{955,
2}, {955, 4}, {955, 7}}, {{956, 2}}, {{957, 1}}, {{960,
6}}, {{961, 8}}, {{963, 2}}, {{965, 2}}, {{966, 1}, {966,
2}}, {{967, 1}}, {{968, 2}}, {{969, 3}, {969, 8}}, {{970, 3}, {970,
6}, {970, 9}}, {{972, 3}}, {{974, 2}}, {{975, 1}}, {{976,
1}, {976, 2}, {976, 6}}, {{979, 4}, {979, 8}, {979, 9}}, {{980,
4}}, {{981, 2}, {981, 4}}, {{982, 3}, {982, 8}}, {{984, 2}, {984,
3}, {984, 6}}, {{987, 1}, {987, 3}}, {{988, 6}}, {{989, 2}, {989,
10}}, {{990, 1}, {990, 3}}, {{991, 3}}, {{992, 4}}, {{994,
1}, {994, 2}, {994, 6}}, {{996, 2}, {996, 3}, {996, 4}, {996,
6}}, {{997, 1}}, {{998, 2}}, {{999, 1}, {999, 9}}, {{1000,
1}, {1000, 2}}}


So, we can see that using the bounds $$1\le\text{n}\le10^3$$ and $$1\le\text{k}\le10$$ we found $$1419$$ solutions. That number is found by using the following Mathematica-code:

In[2]:=Clear["Global*"];
\[Alpha] = 10^3;
\[Beta] = 10^1;
f = Total@*Map[Length];
f[ParallelTable[
If[PrimeQ[n^k + n - 1], {n, k}, Nothing], {n, 1, \[Alpha]}, {k,
1, \[Beta]}] //. {} -> Nothing]

Out[2]=1419


Extending the bounds to $$1\le\text{n}\le10^4$$ and $$1\le\text{k}\le10^2$$ gives $$19235$$ solutions.

I did a search to find solutions for $$\text{n}=107$$. I did find a solution for $$1\le\text{k}\le5\cdot10^3$$ because when $$\text{k}=1400$$ the number $$\text{n}^\text{k}+\text{n}-1$$ is prime.

I also searched for solutions of $$\text{n}$$ and $$\text{k}$$ such that $$\text{k}\ne6\text{m}+5$$ (where $$\text{m}\in\mathbb{N}$$). I did find solutions for $$1\le\text{n}\le10^2$$ and $$1\le\text{k}\le10$$:

In[3]:=Clear["Global*"];
\[Alpha] = 10^2;
\[Beta] = 10^1;
ParallelTable[
If[TrueQ[PrimeQ[n^k + n - 1] && IntegerQ[(k - 5)/6] == False], {n,
k}, Nothing], {n, 1, \[Alpha]}, {k, 1, \[Beta]}] //. {} -> Nothing

Out[3]={{{2, 1}, {2, 2}, {2, 4}, {2, 8}}, {{3, 1}, {3, 2}, {3, 3}, {3,
4}, {3, 8}, {3, 10}}, {{4, 1}, {4, 2}, {4, 3}, {4, 6}, {4, 8}, {4,
9}}, {{5, 2}, {5, 6}, {5, 10}}, {{6, 1}, {6, 2}, {6, 4}, {6,
7}, {6, 10}}, {{7, 1}, {7, 3}}, {{8, 2}, {8, 6}, {8, 10}}, {{9,
1}, {9, 2}, {9, 4}, {9, 7}, {9, 10}}, {{10, 1}, {10, 2}, {10,
3}, {10, 4}, {10, 9}}, {{11, 2}, {11, 8}}, {{12, 1}, {12,
4}}, {{13, 2}, {13, 4}, {13, 10}}, {{15, 1}, {15, 2}, {15, 3}, {15,
9}}, {{16, 1}, {16, 2}, {16, 3}, {16, 4}, {16, 8}, {16,
10}}, {{17, 4}, {17, 8}}, {{18, 3}, {18, 6}}, {{19, 1}, {19,
2}}, {{20, 2}, {20, 4}}, {{21, 1}, {21, 2}, {21, 3}}, {{22,
1}, {22, 7}}, {{23, 4}, {23, 6}}, {{24, 1}, {24, 2}, {24,
10}}, {{25, 3}, {25, 8}}, {{26, 2}, {26, 4}, {26, 10}}, {{27,
1}, {27, 3}, {27, 8}}, {{28, 2}, {28, 4}}, {{29, 10}}, {{30,
1}, {30, 2}, {30, 7}}, {{31, 1}, {31, 2}, {31, 4}, {31, 6}, {31,
10}}, {{33, 3}, {33, 4}}, {{34, 1}, {34, 6}}, {{35, 2}}, {{36,
1}, {36, 3}, {36, 6}}, {{37, 1}, {37, 7}}, {{38, 2}}, {{39,
2}, {39, 3}}, {{40, 1}, {40, 4}, {40, 10}}, {{41, 2}}, {{42,
1}}, {{43, 3}, {43, 4}}, {{44, 2}, {44, 4}}, {{45, 1}, {45,
2}}, {{46, 2}, {46, 3}, {46, 8}}, {{48, 2}}, {{49, 1}, {49,
7}, {49, 9}}, {{50, 2}, {50, 6}, {50, 8}}, {{51, 1}, {51, 3}, {51,
6}, {51, 10}}, {{52, 1}, {52, 3}}, {{53, 2}, {53, 6}}, {{54,
1}, {54, 2}, {54, 4}}, {{55, 1}, {55, 2}, {55, 3}}, {{56, 2}, {56,
6}}, {{57, 1}, {57, 9}}, {{58, 4}}, {{59, 2}, {59, 10}}, {{60,
2}, {60, 6}}, {{61, 7}, {61, 9}}, {{62, 8}}, {{63, 3}, {63,
6}}, {{64, 1}, {64, 2}}, {{65, 2}}, {{66, 1}, {66, 2}, {66,
8}, {66, 9}}, {{68, 2}, {68, 8}}, {{69, 1}, {69, 6}}, {{70,
1}, {70, 2}, {70, 7}, {70, 9}}, {{72, 4}}, {{73, 3}, {73,
6}}, {{74, 8}}, {{75, 1}, {75, 6}}, {{76, 1}, {76, 2}, {76,
6}, {76, 10}}, {{77, 4}, {77, 8}}, {{78, 3}}, {{79, 1}, {79,
9}}, {{81, 3}}, {{82, 1}}, {{83, 2}, {83, 6}}, {{84, 1}, {84,
8}}, {{85, 2}}, {{86, 2}}, {{87, 1}, {87, 3}, {87, 8}}, {{88,
8}}, {{89, 2}, {89, 10}}, {{90, 1}, {90, 10}}, {{91, 1}, {91,
6}}, {{92, 4}}, {{93, 2}, {93, 3}, {93, 4}, {93, 7}}, {{94,
2}, {94, 3}}, {{96, 1}, {96, 2}, {96, 3}, {96, 6}, {96, 10}}, {{97,
1}}, {{98, 4}}, {{99, 1}, {99, 10}}, {{100, 1}, {100, 2}, {100,
3}, {100, 7}, {100, 8}}}
`
• Thank you! Notably, this confirms conjecture $3$ for $k=9$ ($n=172$) Dec 29, 2020 at 14:47
• @MichałZapała You're welcome, I am glad that I could help you. Dec 29, 2020 at 14:48