Bifalgorithm post · Brisk · Chapter 1

How can you quickly judge whether a base-conversion claim is impossible?

How can you quickly judge whether a base-conversion claim is impossible? (Brisk · Chapter 1) — "1. If I told you that the decimal number (i.e., base-10 number)…

Ideas in this flowchart

  • Testing whether base representations can equal the decimal number 2022
  • Base-6 digit validity check
  • The claimed representation 13621₆ is invalid because it contains the digit 6
  • A base-6 number may contain only the digits 0 through 5
  • Base-6 magnitude check
  • In base 6, place values are 1, 6, 36, and 216 rather than 1, 10, 100, and 1000
  • The four-digit number 1413₆ cannot be as large as the four-digit decimal number 2022
  • When the base is smaller, the number must look larger to represent a comparable value
  • Hexadecimal magnitude check
  • Hexadecimal place values, except the one’s place, are larger than the corresponding decimal place values
  • A four-digit hexadecimal number beginning with 8 is much larger than a four-digit decimal number beginning with 2
  • Therefore, 8FA₁₆ cannot equal the decimal number 2022
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How can you quickly judge whether a base-conversion claim is impossible?

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Brisk

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"1. If I told you that the decimal number (i.e., base-10 number) 2022 was equal to 13621₆, would you call me a liar without even having to think too hard? Yes, you should. A number in base-6 can’t have any digits in it other than 0 through 5, and the “number” I tried to give you had a 6 in it.\n\n2. If I told you that the decimal number 2022 was equal to 1413₆, would you call me a liar without even having to think too hard? Yes, you should. Think about it: in base 6, each digit’s place value (except the one’s place) is worth less than it is in base 10. Instead of a ten’s place, hundred’s place, and thousand’s place, we have a woosy six’s place, thirty-six’s place, and two-hundred-and-sixteen’s place. So there’s no way that a number whose base-6 digits are 1, 4, 1, and 3 would be as large as a number whose base-10 digits are 2, something, something, and something. Put another way, if the base is smaller, the number itself has to “look bigger” to have a chance of evening that out.\n\n3. If I told you that the decimal number 2022 was equal to 8FA₁₆, would you call me a liar without even having to think too hard? Yes, you should, because of the mirror reflection of the above logic. Every digit of a hexadecimal number (again, except the one’s place) is worth more than it is in base 10. So a four-digit hex number beginning with an 8 is going to be way bigger than a wimpy four-digit decimal number beginning with 2."

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