# One-dimensional Infrastructures.

One-dimensional infrastructures first appeared in the 1970's in Daniel Shanks' work on real quadratic number fields , a squarefree integer, when he tried to fasten regulator computations. The previous algorithms used continued fraction expansion to obtain the regulator in binary operation, arbitrary. Shanks found out that one can obtain a multiplication like operation, which he dubbed giant steps, as opposed to the baby steps taken by one step in the continued fraction expansion. He described a baby step-giant step method to compute the regulator in binary operations, requiring bytes of storage. His methods were analysed, written up more clearly and extended by various people, including Hendrik Lenstra, Hugh Williams, Johannes Buchmann, Rene Schoof, and many others. Extensions of the method to function fields exist as well, most notably due to the work of Andreas Stein and Renate Scheidler.

I begin with giving an abstract definition of a one-dimensional infrastructure.

Definition (One-dimensional infrastructure).

Let be a real number. A one-dimensional infrastructure of circumference is a pair , where is a finite set and is an injective map.

If you interpret as a circle of circumference (think of it as folding up the real line, such that two numbers whose difference is an integer multiple of are identified), a one-dimensional infrastructure can be seen as a circle with a finite number of dots on it. The map gives the distance between and some element on the circle, whence is called the distance map.

Now one can define two operations on a one-dimensional infrastructure. Due to Shanks' nomenclature, these are called baby steps and giant steps. To define a baby step, let . Then consider the set . It is non-empty as and bounded from below. Moreover, it is discrete as is finite; therefore, exists and , say with . In that case, we define . This gives a bijective map which, in case , has no fixed points. If is interpreted as a circle and identified with , will send each point to the “next one” in positive direction on the circle.

To define giant steps, let . For that, note that is naturally a group, whence we can add and . Now , but in general . But we can use a similar trick as in the baby step case: we jump back to the previous point of . For that, define . It is bounded from above, non-empty and discrete, whence exists with , say for ; then we define . This gives a binary operation which is in general not associative.

But even though, we have

in general, assuming that is small (here, we identify with its smallest non-negative representant). More precisely, we have with , whence the giant step operation is “almost” associative.

#### Finite Cyclic Groups as One-dimensional Infrastructures.

Let be a finite cyclic group of order . For , one can write with ; note that is the discrete logarithm of with respect to . Hence, we get the isomorphism induced by . As , we get the injective map , , turning into a one-dimensional infrastructure.

Let ; then we get and , i.e. baby steps are multiplications by the generator and the giant steps equals the group operation. In particular, this provides an example for giant steps being associative.

Therefore, one-dimensional infrastructures can be seen as generalizations of finite cyclic groups.

#### Remarks.

Finally, we want to sketch some ideas, which will allow generalizing infrastructures to higher dimensions. For that, let be a one-dimensional infrastructure.

First, define the map as follows. For , define . Again, is non-empty, bounded from below and discrete, whence exists and , say for some . Define . Then satisfies and for all .

If would be any other map satisfying , one would obtain another giant step function , . In case comes from a finite cyclic group, as above, would again be the group operation. If this is not the case, and could be two distinct binary operations on . If satisfies for all , we would also have

This shows that our choice of is rather random; we could also define with , or chose such that , with some additional condition to rule out ties. Any other arbitrary choice of is also possible, as long as is satisfied. We will later see that our definition of is exactly the one we obtain in a canonical way if we obtain infrastructures from global fields of unit rank one. We call such maps reduction maps.