The Kaufman-Rickert inequality governs Rademacher sums
The geometric ideas present in the Kaufman-Rickert inequality published 1966, as augmented by Kakutani and later by Kaijser, are powerful enough to completely determine the Sidon constant of the Rademacher system. This seems to have been overlooked in the standard literature; an unjust fate which is to be mended in this small article. Since the Kaufman-Rickert inequality is discussed in the standard reference by Hewitt and Ross [3], this lack of natural attention is surprising. The standard version of the Kaufman-Rickert inequality says that