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Will Jagy
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Numbers integrally represented by a ternary cubic form

This is something I made up years ago when Kap got interested in the "tribonacci" numbers. A few weeks ago, I put this at MSE but no takers. Oh, years ago I asked some guy at MSRI about this, he muttered something about norm forms and laughed at me.

Given integers $a,b,c,$ and cubic form $$ f(a,b,c) = a^3 + b^3 + c^3 + a^2 b - a b^2 + 3 a^2 c - a c^2 + b^2 c - b c^2 - 4 a b c $$ $$ f(a,b,c) = \det \left( \begin{array}{ccc} a & b & c \\\ c & a + c & b + c \\\ b + c & b + 2 c & a + b + 2 c \end{array} \right) . $$ what primes $p$ can be integrally represented as $$ p = f(a,b,c)? $$

(A): I think it is all primes $(p| 11) = -1 ,$ and all $p = u^2 + 11 v^2$ in integers, but not any $q = 3 u^2 + 2 u v + 4 v^2.$ Note that, if $-p$ is represented, so is $p.$

(B): I also suspect that if prime $q = 3 u^2 + 2 u v + 4 v^2$ and $f(a,b,c) \equiv 0 \pmod q,$ then all three $a,b,c \equiv 0 \pmod q,$ and $f(a,b,c) \equiv 0 \pmod {q^3}.$

Note that if $f$ integrally represents both $m,n$ then it represents $mn.$ That is because $f(a,b,c) = \det(aI + b X + c X^2),$ where $$ X = \left( \begin{array}{ccc} 0 & 1 & 0 \\\ 0 & 0 & 1 \\\ 1 & 1 & 1 \end{array} \right) $$ Then $X^3 = X^2 + X + I$ and $X^4 = 2 X^2 + 2 X + I.$

If all suspicions are correct, we can correctly describe all numbers integrally represented by this polynomial: positive or negative are unimportant, most prime factors are unimportant, all that matters is that every exponent of a prime factor $q = 3 u^2 + 2 u v + 4 v^2$ must be divisible by 3.

I should have done this last time: most of the class field part has already been done, by Hudson and Williams (1991), Theorem 1 and Table 1 on page 134. You get my version of the polynomial by negating their variable $x.$

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       p           a           b           c 
       2           0           1           1
       7           0         -11           6
      11           0          -3           2
      13           0          -1           2
      17          -1           0           2
      19           1           2           4
      29           0          -7           4
      41           0           3           2
      43           0           4          -1
      47           0           5          -2
      53           0           1           4
      61           0          46         -25
      73           2         -36          19
      79           0           3           4
      83           0          24         -13
     101          -1          12          -6
     103           0          15          -8
     107           1          -9           5
     109           1           2           6
     127           1          -2           4
     131           1           7          -3
     139           1          -6           4
     149          -1           4           2
     151           0         -20          11
     163           0           5           2
     167          -1           1           5
     173           0           6          -1
     193           1         -52          28
     197           0           9          -4
     199          -1           5           1
     211           0         -12           7
     227          -2           0           5
     233           0         -16           9
     239           0          -6           5
     241           0          -4           5
     257           0          -1           6
     263           2           4           9
     269          -1           0           6
     271           2           8          -3
     277           1          -7           5
     281           0           2           7
     283          -1           2           6
     293          -1          -8           6
     307           2          -1           6
     311           0           5           6
     337          -2           5           2
     347           1           7           5
     349           0          19         -10
     359          -1           9          -3
     373           2           5          10
     397           1          -1           7
     401           0         -68          37
     409           3         -77          41
     419           0          -7           6
     421           0           7           2
     431           1         -14           8
     439           0           8          -1
     457           0           1           8
     461           0          -2           7
     479           1          -8           6
     491           0           7           4
     499           0          13          -6
     503          -1         -36          20
     523           0           9          -2
     541           2         -12           7
     547           1         -11           7
     557          -1          25         -13
     563          -2         -11           8
     569           0           8           1
     571           1          -3           7
     587           0         -29          16
     593           3         -25          13
     599          -1           0           8
     601           0           7           6
     607           0          11          -4
     613           0           4           9
     617           2          -1           8
     659           0           8           3
     673           0          -6           7
     677           0         -17          10
     683          -1           4           8
     701           2          13          -6
     733           1          10          -2
     739          -1          14          -6
     743          -2           1           8
     757           0          81         -44
     761          -1           8           2
     769           0         -25          14
     773          -1           7           5
     787           2           5          12
     809          -1         -10           8
     811          -4           0           7
     821          -1           3           9
     827           2          10           7
     853           0         -11           8
     857          -2           3           8
     863           0           9           2
     877          -2         -15          10
     883           0         -14           9
     887           2          -3           8
     907           0          -5           8
     911           0           8           7
     919           0          -2           9
     929           1           7          11
     937           3           8          14
     941           3          -1           9
     953          -1           6           8
     967           1          13          -5
     991           1         -35          19
     997          -3           7           3

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Will Jagy
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