The four-dimensional summability methods of Euler and Borel are studied as mappings from absolutely convergent double sequences into themselves. Also the following Tauberian results are proved: if x = (x(m,n)) is a double sequence that is mapped into l(2) by the four-dimensional Borel method and the double sequence x satisfies Sigma(infinity)(m-0) Sigma(infinity)(n=0)vertical bar Delta 10x(m,n)vertical bar root mn < infinity and Sigma(infinity)(m=0) Sigma(infinity)(n=0)vertical bar Delta 01x(m,n)vertical bar root mn < infinity, the x itself is in l(2).
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Hamilton H. J., 1936, DUKE MATH J, V2, P29, DOI DOI 10.1215/S0012-7094-36-00204-1