The image of 0 is the zero of ℝ, and for n ≥ 1 the image is the last value of the iterated sum, a map into ℝ; ι(n) is the value of a map into ℤ.
Let . By The Integers (v), is a map from to , so its value at lies in by Maps between Classes: Values, Pairs, Extensionality, Restrictions, Constant Maps and Preimages (i). This proves (i).
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