Einar claims, “If I know \(\log(2)\) and \(\log(3)\), I can use logarithm properties to find the common logarithm of every composite number from \(1\) through \(20\) exactly.”
1. Decide whether the claim is true. Justify your answer with a counterexample or a general argument.
2. Using \(\log(2)\), \(\log(3)\), and \(\log(10)=1\), which natural numbers from \(1\) through \(20\) have logarithms that can be determined?
3. Which additional prime-number logarithms are needed to determine the logarithms of every natural number from \(1\) through \(20\)?
Hints
- Use prime factorization to identify which logarithms are needed.
- Consider whether \(\log(10)\) lets you recover the logarithm of another prime from a product you already know.
- Check which prime factors appear among the natural numbers through \(20\).
Solution
1. The claim is false. For example, \(14=2\cdot7\), so \(\log(14)=\log(2)+\log(7)\). The value of \(\log(7)\) is not provided and cannot be obtained from the stated values using logarithm properties alone.
2. Since \(\log(5)=\log(10)-\log(2)\), logarithms can be determined for numbers whose prime factorizations use only \(2\), \(3\), and \(5\). From \(1\) through \(20\), these are \(1,2,3,4,5,6,8,9,10,12,15,16,18,\) and \(20\).
3. The missing primes are \(7,11,13,17,\) and \(19\). Knowing their logarithms would allow every integer from \(1\) through \(20\) to be handled by prime factorization.
Answer
1. The claim is false; for example, finding \(\log(14)\) requires \(\log(7)\).
2. \(1,2,3,4,5,6,8,9,10,12,15,16,18,20\)
3. \(7,11,13,17,19\)