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 A053343 Semiprimes of the form pq where p < q and p + q - 1 is prime. 1
 15, 33, 35, 51, 65, 77, 87, 91, 95, 119, 123, 143, 161, 177, 185, 209, 213, 215, 217, 221, 247, 259, 287, 303, 321, 329, 335, 341, 371, 377, 395, 403, 407, 411, 427, 437, 447, 469, 473, 485, 511, 515, 527, 533, 537, 545, 551, 573, 581, 591, 611, 629, 635 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Squarefree terms of A050530 with 2 prime divisors. All terms are odd. - Muniru A Asiru, Aug 29 2017 LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..1000 HacĂ¨ne Belbachir, Oussama Igueroufa, Combinatorial interpretation of bisnomial coefficients and Generalized Catalan numbers, Proceedings of the 1st International Conference on Algebras, Graphs and Ordered Sets (ALGOS 2020), hal-02918958 [math.cs], 47-54. FORMULA n=pq such that n-phi(n) = pq-(p-1)(q-1) = p+q-1 is prime. MATHEMATICA With[{nn=70}, Take[Times@@@Select[Subsets[Prime[Range[nn]], {2}], PrimeQ[Total[#] - 1] &]//Union, nn]] (* Vincenzo Librandi, Aug 23 2017 *) PROG (PARI) list(lim)=my(v=List()); forprime(p=5, lim\3, forprime(q=3, min(lim\p, p-2), if(isprime(p+q-1), listput(v, p*q)))); Set(v) \\ Charles R Greathouse IV, Aug 23 2017 (GAP) A053343:=List(Filtered(Filtered(List(Filtered(List([1..10^5], Factors), i->Length(i)=2), Set), j->Length(j)=2), i->IsPrime(Sum(i)-1)), Product); # Muniru A Asiru, Aug 29 2017 CROSSREFS Subsequence of A291318. Cf. A050530, A290434, A290435. Sequence in context: A337984 A050384 A142862 * A068081 A089967 A064900 Adjacent sequences:  A053340 A053341 A053342 * A053344 A053345 A053346 KEYWORD nonn,easy AUTHOR Labos Elemer, Jan 05 2000 EXTENSIONS New name from Vincenzo Librandi Aug 23 2017 STATUS approved

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Last modified June 19 21:30 EDT 2021. Contains 345151 sequences. (Running on oeis4.)