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 A117065 Primes that are not the sum of 3 pentagonal numbers. 6
 19, 31, 43, 67, 89, 101, 113, 131, 229, 241, 277, 359, 383, 491, 523, 619, 631, 643, 701, 761, 1321, 1381, 1621, 2221, 2861 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS 5 is the only prime pentagonal number; every greater pentagonal number A000326(n) = n(3n-1)/2 is either divisible by n/2 or (3n-1)/2. Every number is the sum of 5 pentagonal numbers, hence every prime is the sum of 5 pentagonal numbers. There are an infinite number of primes which are the sum of two pentagonal numbers, the subset of primes which are the sum of two pentagonal numbers in exactly two different ways begins {211, 853, 1259, 1427, 1571, 2297, 2351}. The sum may include the pentagonal number 0. Hence this sequence does not have any primes that are the sum of two positive pentagonal numbers. The sequence is probably finite. There are no other primes < 59900. - T. D. Noe, Apr 19 2006 The next term, if it exists, is greater than 160000000. - Jack W Grahl, Jul 10 2018 a(26) > 10^11, if it exists. - Giovanni Resta, Jul 13 2018 LINKS R. K. Guy, Every number is expressible as the sum of how many polygonal numbers?, Amer. Math. Monthly 101 (1994), 169-172. FORMULA A000040 INTERSECT A003679. MATHEMATICA nn=201; pen=Table[n(3n-1)/2, {n, 0, nn-1}]; ps=Prime[Range[PrimePi[pen[[ -1]]]]]; Do[n=pen[[i]]+pen[[j]]+pen[[k]]; If[n<=pen[[ -1]]&&PrimeQ[n], ps=DeleteCases[ps, _?(#==n&)]]], {i, nn}, {j, i, nn}, {k, j, nn}]; ps (* T. D. Noe, Apr 19 2006 *) CROSSREFS Cf. A000040, A000326, A003679, A064826. Sequence in context: A040068 A096787 A104006 * A006035 A104485 A276569 Adjacent sequences:  A117062 A117063 A117064 * A117066 A117067 A117068 KEYWORD more,hard,nonn AUTHOR Jonathan Vos Post, Apr 17 2006 EXTENSIONS More terms from T. D. Noe, Apr 19 2006 STATUS approved

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Last modified May 22 14:32 EDT 2019. Contains 323480 sequences. (Running on oeis4.)