This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A198327 Numbers n such that n and n-2 are semiprimes. 2
 6, 35, 51, 57, 87, 93, 95, 121, 123, 143, 145, 161, 185, 187, 203, 205, 215, 217, 219, 221, 237, 249, 267, 289, 291, 301, 303, 305, 321, 323, 329, 341, 393, 395, 413, 415, 417, 447, 453, 471, 473, 517, 519, 529, 535, 537, 545, 553, 581, 583, 591, 635, 669, 671 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Omega(a(n)) = Omega[a(n) - Omega(a(n))] because  Omega(a(n)) = 2, and a(n) - 2 is semiprime => this sequence is a subset of  A200925. LINKS Harvey P. Dale, Table of n, a(n) for n = 1..1000 Eric Weisstein's World of Mathematics, Semiprime FORMULA a(n) = A092207(n) + 2. MATHEMATICA PrimeFactorExponentsAdded[n_] := Plus @@ Flatten[Table[ #[[2]], {1}] & /@ FactorInteger[n]]; Select[ Range[ 671], PrimeFactorExponentsAdded[ # ] == PrimeFactorExponentsAdded[ # - 2] == 2 &] SemiPrimeQ[n_Integer] := If[Abs[n] < 2, False, (2 == Plus @@ Transpose[FactorInteger[Abs[n]]][[2]])]; Select[Range[1000], SemiPrimeQ[#] && SemiPrimeQ[# - 2] &] (* T. D. Noe, Nov 27 2011 *) #[[3, 1]]&/@Select[Partition[Table[{n, PrimeOmega[n]}, {n, 700}], 3, 1], #[[1, 2]]==#[[3, 2]]==2&] (* From Harvey P. Dale, Dec 10 2011 *) CROSSREFS Cf. A001222, A092207, A200925. Sequence in context: A145000 A209182 A046025 * A009583 A033578 A101077 Adjacent sequences:  A198324 A198325 A198326 * A198328 A198329 A198330 KEYWORD nonn AUTHOR Michel Lagneau, Nov 25 2011 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Transforms | Puzzles | Hot | Classics
Recent Additions | More pages | Superseeker | Maintained by The OEIS Foundation Inc.

Content is available under The OEIS End-User License Agreement .

Last modified May 18 07:10 EDT 2013. Contains 225419 sequences.