There are a total of five known Fermat primes, which also include 3, 17, 257, and 65537. The sum of the first 3 Fermat primes, 3, 5 and 17, yields 25 or 52, while 257 is the 55th prime number. Combinations from these 5 Fermat primes generate 31 polygons with an odd number of sides that can be constructed purely with a compass and straight-edge, which includes the five-sided regular pentagon. Apropos, 31 is also equal to the sum of the maximum number of areas inside a circle that are formed from the sides and diagonals of the first five 
  
    
      
        n
      
    
    {\displaystyle n}
  -sided polygons, and equal to the maximum number of areas formed by a six-sided polygon; per Mosers circle problem.The number 5 is the fifth Fibonacci number, being 2 plus 3. It is the only Fibonacci number that is equal to its position aside from 1, which is both the first and second Fibonacci numbers. Five is also a Pell number and a Markov number, appearing in solutions to the Markov Diophantine equation: (1, 2, 5), (1, 5, 13), (2, 5, 29), (5, 13, 194), (5, 29, 433), ... (OEIS: A030452 lists Markov numbers that appear in solutions where one of the other two terms is 5).
Trolamine salicylate (Aspercreme, Aspergel) is an organic compound which is the salt formed between triethanolamine and salicylic acid. It is used as an ingredient in sunscreens, analgesic creams, and cosmetics. The salicylic acid portion contributes to both the sun protection effect (by absorbing UVB radiation) and to the analgesic effect. The triethanolamine neutralizes the acidity of the salicylic acid. One benefit of this topical analgesic is that it has no odor, in contrast to other topical analgesics such as menthol. The US Food and Drug Administration has not reviewed any of the over-the-counter products listed in the Daily Med database that contain trolamine salicylate. Also, the producers of trolamine salicylate products have not provided evidence to the FDA in support of claims that this chemical is directly absorbed through the skin into underlying tissue. Due to health concerns, in 2019 the FDA issued a proposed rule classifying the sunscreen use of trolamine salicylate as "not generally recognized as safe and effective." One study reported that trolamine salicylate does penetrate into, and persist within, underlying muscle tissue. The test subjects used either the trolamine salicylate product or a placebo while engaging in an exercise regimen designed to induce muscle soreness.