How many primes below 300 are followed
by a semiprime number?
List them.
There are precisely 9 prime-semiprime pairs, and these are furnished hereunder as follows:
Prime Semiprime Why semiprime?
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3 4 4 = 2*2
5 6 6 = 2*3
13 14 14 = 2*7
37 38 38 = 2*19
61 62 62 = 2*31
73 74 74 = 2*37
157 158 158 = 2*79
193 194 194 = 2*97
277 278 278 = 2*139
Methodology
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Wikipedia provides a list of first 1000 prime numbers. So I considered the list of prime numbers from 2 to 293 inclusively. This invariably shortened the semiprime candidates.
It was then very easy to pry them out as each of the valid ones had the form 2*p, where p is a prime number.
Edited on August 29, 2022, 10:01 pm
Puzzle Solution
