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Mathematical rule
The shift rule is a mathematical rule for sequences and series.
For sequences, the rule states that if is a sequence, then it converges if and only if also converges, and in this case both sequences always converge to the same number.
For series, the rule states that the series converges to a number if and only if converges.
References
Ueltschi, Daniel (2011), Analysis –MA131 (PDF), University of Warwick, p. 31.