In 1895, French mathematician Émile Lemoine proposed that :
2n + 1 = p + 2q always has a solution in primes p and q (not necessarily distinct) for n > 2.
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In plain language :
All odd integers greater than 5 can be represented as the sum of an odd prime number and an even semiprime.* see notes
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e.g.
13=3+(2×5)
19=5+(2×7)
Although the conjecture has been computationally checked up to at least 1010, there is currently no proof (or disproof).
More info at Wikipedia
Notes:
[1] The conjecture is also called Levy's Conjecture which was (re)proposed in 1963.
[2] An 'even semiprime' is any even number which is the product of two prime numbers multiplied together.