The beginnings of REDTEN go back to 1983, when a summer project was begun with the goal of implementing a tensor algebra system in MAPLE. After many difficulties this project was abandoned, but was later taken up again for a Masters thesis, this time using muMATH as the base system.
muMATH was a small, relatively simple computer algebra system, running on IBM PC's and related INTEL-88 machines. As a LISP-based system, and with source code available for all but the kernel functions, it was particularly easy to introduce new data structures and functions, and, where necessary, re-define existing ones. Thus, in a relatively short time a basic working tensor system was produced. After a somewhat longer period a more advanced and capable system was available, with essentially all of the major features of REDTEN today.
From the outset, it was clear that a new system should implement tensor algebra, and not just a few operations with tensors. It was intended that the user be able to enter expressions and see results as close to the conventional form as was possible with a normal terminal screen and keyboard. Rather than calling a function to add two tensors, for example, the user would simply type the names and associated indices, and assign the result to a new tensor. If the sum involved three objects, the user would just add the third name; in other systems it might be necessary to break this operation into two parts, involving two sums and one intermediate object. It was this ``serial construction'' that was particularly to be avoided.
To accomplish this goal generally requires modifying the system parser to accept new operators. In muMATH this possibility is designed into the parser, and is therefore extremely simple. With a new assignment operator (in muTENSOR it was ``::'', in REDTEN it is ``=='') a new function is installed to handle the input: it receives the left and right hand sides of the assignment. Since the data structure representing algebraic expressions is known, it is a straightforward matter for the new system function to treat the expression as a tensor expression, and to produce the desired result.
As muTENSOR advanced, the amount of code involved soon exceeded that in the basic muMATH kernel. Since muMATH was capable of using only 256 Kilobytes of memory (even on machines with larger amounts of memory), the amount of working space (where algebraic calculations are carried out and results stored) gradually decreased. Eventually, there was almost no room left to introduce new functions, and large calculations could not be performed due to the space required to hold all the intermediate results in core. This prompted the development of a ``virtual memory'' sub-system, which allowed the storage of the components of a tensor in secondary storage, such as a hard-disk or ram-disk. A needed component was read in from the disk, used, discarded and the results of the computation possibly written to disk, all automatically; generally the time required to retrieve the component was much less than the time required to do algebra with it, so that the overhead was negligible. This allowed much larger problems to be successfully tackled, such as showing that the Kerr metric is vacuum, which required about 20 minutes on an IBM AT.
muTENSOR was demonstrated or described at several General Relativity conferences, and as a result, a moderately large group of users developed in several countries. However, the small amount of working space, even with the virtual memory sub-system, and the very simple algorithms used to implement algebra in muMATH made it increasingly obvious that a larger more capable base system was needed if further advancement was to be made.
In late 1985 it was determined that REDUCEhttp://??? would provide the power needed for an improved version of muTENSOR, to be called REDTEN. Since REDUCE is also LISP-based, the transportation of the code to the REDUCE environment was relatively simple, the major difficulties being in the interaction with the I/O system and the internal data structure of expressions, which although similar in concept to those of muMATH, differed in detail. Otherwise, most of the code was simply translated directly from muSIMP (the base lisp of muMATH) to RLISP (the extended syntax of Standard Lisp) in which REDUCE is written. To this day there is code in REDTEN with an odd ``flavour'' due to the original influence of muSIMP.
For a time the two systems evolved in parallel, each receiving the same upgrades and bug fixes as the other. In REDTEN a virtual memory system was unnecessary, while at the same time more code could be developed to expand the system. Thus, REDTEN gradually became more advanced, with several new features not available in muTENSOR (such as frame and spinor packages). It was found the the REDTEN system was very capable and met all expectations.
An attempt was again made to implement a tensor-algebra system in MAPLE, using muTENSOR as a model. This system, called MapleTensor, was partially successful, but met difficulties with interacting with the MAPLE I/O system and with accessing the internal data structures used for expressions. It therefore lacked the same ``feel'' as muTENSOR and REDTEN and was not as convenient to use; support for MapleTensor has since been dropped.
In 1988(??) muMATH was discontinued, and a new product, DERIVE, was introduced by the Soft Warehouse. Unfortunately, DERIVE, as a much simpler system, does not include a programming environment. Hence, muTENSOR became an orphaned system, and its development has halted, while that of REDTEN has continued. Although a successor to muMATH may become available in the future, many of todays personal computers are capable of running REDUCE (and therefore REDTEN), so that the need for a small compact system is less of an issue.
Currently REDTEN is at version v4.0, and is continuing to evolve. Many improvements have been made in the past few years, and the kernel of the system has reached a relatively stable configuration. Recent improvements also include support for REDUCE 3.5 (the current version of REDUCE) and a basic clean-up of the code. It is hoped that REDTEN will soon become a part of the REDUCE network-library of contributed programs, and therefore more widely available.