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Current circulating in one antenna generally induces a voltage across the feedpoint of nearby antennas or antenna elements. The mathematics presented below are useful in analyzing the electrical behaviour of antenna arrays, where the properties of the individual array elements (such as half wave dipoles) are already known. If those elements were widely separated and driven in a certain amplitude and phase, then each would act independently as that element is known to. However, because of the mutual interaction between their electric and magnetic fields due to proximity, the currents in each element are not simply a function of the applied voltage (according to its driving point impedance), but depend on the currents in the other nearby elements. Note that this now is a near field phenomenon which could not be properly accounted for using the Friis transmission equation for instance.

What are the mathmatic useful towards?
Answer: electrical behaviour
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Old English is one of the West Germanic languages, and its closest relatives are Old Frisian and Old Saxon. Like other old Germanic languages, it is very different from Modern English and difficult for Modern English speakers to understand without study. Old English grammar is quite similar to that of modern German: nouns, adjectives, pronouns, and verbs have many inflectional endings and forms, and word order is much freer. The oldest Old English inscriptions were written using a runic system, but from about the 9th century this was replaced by a version of the Latin alphabet.

In what century did the Latin alphabet replace the runic system in Old English writing?
Answer: 9th
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White-box testing (also known as clear box testing, glass box testing, transparent box testing and structural testing, by seeing the source code) tests internal structures or workings of a program, as opposed to the functionality exposed to the end-user. In white-box testing an internal perspective of the system, as well as programming skills, are used to design test cases. The tester chooses inputs to exercise paths through the code and determine the appropriate outputs. This is analogous to testing nodes in a circuit, e.g. in-circuit testing (ICT).

What is another term used for White-box testing?
Answer:
clear box testing