frobeniusNu -- computes the largest power of an ideal not contained in a specified Frobenius power

Consider an element $f$ of a polynomial ring $R$ over a finite field of characteristic $p$, and an ideal $J$ of this ring. If $f$ is contained in the radical of $J$, then the command frobeniusNu(e,f,J) outputs the maximal exponent $n$ such that $f^{ n}$ is not contained in the $p^e$-th Frobenius power of $J$. More generally, if $I$ is an ideal contained in the radical of $J$, then frobeniusNu(e,I,J) outputs the maximal integer exponent $n$ such that $I^n$ is not contained in the $p^e$-th Frobenius power of $J$.

These numbers are denoted $\nu_f^J(p^e)$ and $\nu_I^J(p^e)$, respectively, in the literature, and were originally defined in the paper $F$-thresholds and Bernstein-Sato Polynomials, by Mustaţă, Takagi, and Watanabe.

If $f$ or $I$ is zero, then frobeniusNu returns 0; if $f$ or $I$ is not contained in the radical of $J$, frobeniusNu returns infinity.

When the third argument is omitted, the ideal $J$ is assumed to be the homogeneous maximal ideal of $R$.

It is well known that if $q=p^e$ for some nonnegative integer $e$, then $\nu_I^J(qp) = \nu_I^J(q) p + L$, where the error term $L$ is nonnegative, and can be explicitly bounded from above in terms of $p$ and the number of generators of $I$ and $J$ (e.g., $L$ is at most $p-1$ when $I$ is principal). This implies that when searching for frobeniusNu(e,I,J), it is always safe to start at $p$ times frobeniusNu(e-1,I,J), and one need not search too far past this number, and suggests that the most efficient way to compute frobeniusNu(e,I,J) is to compute, successively, frobeniusNu(i,I,J), for i = 0,\ldots,e. This is indeed how the computation is done in most cases.

If $M$ is the homogeneous maximal ideal of $R$ and $f$ is an element of $R$, the numbers $\nu_f^M(p^e)$ determine and are determined by the $F$-pure threshold of $f$ at the origin. Indeed, $\nu_f^M(p^e)$ is $p^e$ times the truncation of the non-terminating base $p$ expansion of fpt($f$) at its $e$^{th} spot. This fact is used to speed up the computations for certain polynomials whose $F$-pure thresholds can be quickly computed via special algorithms, namely diagonal polynomials, binomials, forms in two variables, and polynomials whose factors are in simple normal crossing. This feature can be disabled by setting the option UseSpecialAlgorithms (default value true) to false.

The valid values for the option ContainmentTest are FrobeniusPower, FrobeniusRoot, and StandardPower. The default value of this option depends on what is passed to frobeniusNu. Indeed, by default, ContainmentTest is set to FrobeniusRoot if frobeniusNu is passed a ring element $f$, and is set to StandardPower if frobeniusNu is passed an ideal $I$. We describe the consequences of setting ContainmentTest to each of these values below.

First, if ContainmentTest is set to StandardPower, then the ideal containments checked when computing frobeniusNu(e,I,J) are verified directly. That is, the standard power $I^n$ is first computed, and a check is then run to see if it is contained in the $p^e$-th Frobenius power of $J$.

Alternately, if ContainmentTest is set to FrobeniusRoot, then the ideal containments are verified using Frobenius Roots. That is, the $p^e$-th Frobenius root of $I^n$ is first computed, and a check is then run to see if it is contained in $J$. The output is unaffected, but this option often speeds up computations, specially when a polynomial or principal ideal is passed as the second argument.

Finally, when ContainmentTest is set to FrobeniusPower, then instead of producing the invariant $\nu_I^J(p^e)$ as defined above, frobeniusNu instead outputs the maximal integer $n$ such that the $n$^{th} (generalized) Frobenius power of $I$ is not contained in the $p^e$-th Frobenius power of $J$. Here, the $n$^{th} Frobenius power of $I$, when $n$ is a nonnegative integer, is as defined in the paper Frobenius Powers by Hernández, Teixeira, and Witt, which can be computed with the function frobeniusPower, from the TestIdeals package. In particular, frobeniusNu(e,I,J) and frobeniusNu(e,I,J,ContainmentTest=>FrobeniusPower) need not agree. However, they will agree when $I$ is a principal ideal.

The function frobeniusNu works by searching through the list of potential integers $n$ and checking containments of $I^n$ in the specified Frobenius power of $J$. The way this search is approached is specified by the option Search, which can be set to Binary (the default value) or Linear.

The option AtOrigin (default value true) can be turned off to tell frobeniusNu to effectively do the computation over all possible maximal ideals $J$ and take the minimum.

The option ReturnList (default value false) can be used to request that the output be not only $\nu_I^J(p^e)$, but a list containing $\nu_I^J(p^i)$, for $i=0,\ldots,e$.

Alternatively, the option Verbose (default value false) can be used to request that the values $\nu_I^J(p^i)$ ($i=0,\ldots,e$) be printed as they are computed, to monitor the progress of the computation.