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1973.
![s_n^((lambda))(x)=[h(t)]^lambdas_n(x),](/images/equations/CrossSequence/NumberedEquation1.svg)
is a Sheffer sequence, 
is invertible, and 
ranges over the real numbers is called a Steffensen
sequence. If 
is an associated Sheffer sequence, then 
is called a cross sequence.
