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Composition of two arithmetic functions

Source: Romania JBMO TST 2024 Day 3 P3

July 31, 2024
number theoryArithmetic Functions

Problem Statement

Let σ(⋅)\sigma(\cdot) denote the divisor sum function and d(⋅)d(\cdot) denote the divisor counting function. Find all positve integers nn such that σ(d(n))=n.\sigma(d(n))=n.
Andrei Bâra
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