# frobeniusPower -- compute a (generalized) Frobenius power of an ideal

## Synopsis

• Usage:
frobeniusPower(n, I)
frobeniusPower(t, I)
• Inputs:
• n, an integer, nonnegative
• t, , nonnegative
• I, an ideal, in a ring of characteristic $p > 0$
• Optional inputs:
• FrobeniusPowerStrategy => , default value Naive, selects the strategy for frobeniusPower
• FrobeniusRootStrategy => , default value Substitution, selects the strategy for internal frobeniusRoot calls
• Outputs:
• an ideal, the n^{th} or t^{th} Frobenius power of I

## Description

If $I$ is an ideal in a ring of positive characteristic $p$, then frobeniusPower(t, I) computes the generalized Frobenius power $I^{[t]}$, as introduced by Hernandez, Teixeira, and Witt. If the exponent is a power of the characteristic, this is just the usual Frobenius power.

 i1 : R = ZZ/5[x,y]; i2 : I = ideal(x, y); o2 : Ideal of R i3 : frobeniusPower(125, I) 125 125 o3 = ideal (x , y ) o3 : Ideal of R

If $n$ is an arbitrary nonnegative integer, then write the base $p$ expansion of $n$ as follows: $n = a_0 + a_1 p + a_2 p^2 + ... + a_r p^r$. Then the $n^{th}$ Frobenius power of $I$ is defined as follows: $I^{[n]} = (I^{a_0})(I^{a_1})^{[p]}(I^{a_2})^{[p^2]}\cdots(I^{a_r})^{[p^r]}$.

 i4 : R = ZZ/3[x,y]; i5 : I = ideal(x, y); o5 : Ideal of R i6 : adicExpansion(3, 17) o6 = {2, 2, 1} o6 : List i7 : J1 = I^2*frobenius(1, I^2)*frobenius(2, I); o7 : Ideal of R i8 : J2 = frobeniusPower(17, I); o8 : Ideal of R i9 : J1 == J2 o9 = true

If $t$ is a rational number of the form $t = a/p^e$, then $I^{[t]} = (I^{[a]})^{[1/p^e]}$.

 i10 : R = ZZ/5[x,y,z]; i11 : I = ideal(x^50*z^95, y^100 + z^27); o11 : Ideal of R i12 : frobeniusPower(4/5^2, I) 4 4 3 8 2 12 16 o12 = ideal (z , y z , y z , y z, y ) o12 : Ideal of R i13 : frobeniusRoot(2, frobeniusPower(4, I)) 4 4 3 8 2 12 16 o13 = ideal (z , y z , y z , y z, y ) o13 : Ideal of R

If $t$ is an arbitrary nonegative rational number, and \{$t_n$\} = \{$a_n/p^{e_n}$\}\ is a sequence of rational numbers converging to $t$ from above, then $I^{[t]}$ is the largest ideal in the increasing chain of ideals \{$I^{[t_n]}$\}.

 i14 : p = 7; i15 : R = ZZ/p[x,y]; i16 : I = ideal(x^50, y^30); o16 : Ideal of R i17 : t = 6/19; i18 : expon = e -> ceiling(p^e*t)/p^e; -- a sequence converging to t from above i19 : print \ apply(6, i -> frobeniusPower(expon i, I)); 50 30 ideal (x , y ) 12 7 8 14 4 21 ideal (y , x y , x y , x ) 9 2 8 7 5 8 4 14 15 ideal (y , x y , x y , x y , x y, x ) 9 8 7 4 14 ideal (y , x*y , x y , x ) 9 8 7 4 14 ideal (y , x*y , x y , x ) 9 8 7 4 14 ideal (y , x*y , x y , x ) i20 : frobeniusPower(t, I) 9 8 7 4 14 o20 = ideal (y , x*y , x y , x ) o20 : Ideal of R

The option FrobeniusPowerStrategy controls the strategy for computing the generalized Frobenius power $I^{[t]}$. The two valid options are Safe and Naive, and the default strategy is Naive.

The option FrobeniusRootStrategy is passed to internal frobeniusRoot calls.