The phrases this is and these are are often seen at the beginning of clauses or sentences. The demonstrative pronouns this and these refer back to a word or idea in the previous clause or sentence.

Runic artefacts are both textual sources and archaeological artefacts. Because of this it will be important to classify runic artefacts according to both function and inscription.

However, demonstrative pronouns can reduce writing clarity when they are used in the following situations: 1) at the beginning of sentences; 2) in sentences in which several topics are covered; 3) in more complicated sentences containing several clauses. For example, in the second sentence, it is not clear which topic the pronoun this refers to.

The algorithms discussed in this paper, however theoretically fast, must also be implemented in an efficient way, and certain difficulties will be encountered. This is discussed in the framework of future exascale computers.

The word this could refer to the difficulties, the need for implementation of an efficient algorithm, or the theoretical speed of the algorithms. The problem is easily fixed by turning the demonstrative pronoun into a demonstrative adjective modifying the noun. In other words, simply add or repeat the relevant noun, like this:

The algorithms discussed in this paper, however theoretically fast, must also be implemented in an efficient way, and certain difficulties will be encountered. These difficulties are discussed in the framework of future exascale computers.