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 A046315 Odd semiprimes: odd numbers divisible by exactly 2 primes (counted with multiplicity). 64
 9, 15, 21, 25, 33, 35, 39, 49, 51, 55, 57, 65, 69, 77, 85, 87, 91, 93, 95, 111, 115, 119, 121, 123, 129, 133, 141, 143, 145, 155, 159, 161, 169, 177, 183, 185, 187, 201, 203, 205, 209, 213, 215, 217, 219, 221, 235, 237, 247, 249, 253, 259, 265, 267, 287, 289 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The number of odd semiprimes < 10^n: 1, 19, 204, 1956, 18245, 168497, 1555811, 14426124, 134432669, ...; see A085770. - Robert G. Wilson v, Aug 25 2011 In general, the prime factors, p, of a(n) are given by: p = sqrt(a(n) + (k/2)^2) +- (k/2) where k is the positive difference of the prime factors.  Equivalently, p = (1/2)( sqrt(4a(n) + k^2) +- k ). - Wesley Ivan Hurt, Jun 28 2013 LINKS Zak Seidov and K. D. Bajpai, Table of n, a(n) for n = 1..10000 (first 1956 terms from Zak Seidov) EXAMPLE 15 is in the sequence because it is odd number and also 15 = 3 * 5, which is semiprime. - K. D. Bajpai, Jul 05 2014 39 is in the sequence because it is odd number and also 39 = 3 * 13, which is semiprime. - K. D. Bajpai, Jul 05 2014 MAPLE A046315 := proc(n) option remember; local r; if n = 0 then RETURN(0) fi; for r from A046315(n - 1) + 1 do    if r mod 2 = 1 and numtheory[bigomega](r) = 2    then RETURN(r) fi od end: seq(A046315(n), n=1..56); # Peter Luschny, Feb 15 2011 MATHEMATICA Reap[Do[If[Total[FactorInteger[n]][] == 2, Sow[n]], {n, 1, 400, 2}]][[2, 1]] (* Zak Seidov *) fQ[n_] := Plus @@ Last /@ FactorInteger@ n == 2; Select[2 Range@ 150 - 1, fQ] (* Robert G. Wilson v, Feb 15 2011 *) Select[Range[5, 301, 2], PrimeOmega[#]==2&] (* Harvey P. Dale, May 22 2015 *) PROG (PARI) list(lim)=my(u=primes(primepi(lim\3)), v=List(), t); for(i=2, #u, for(j=i, #u, t=u[i]*u[j]; if(t>lim, break); listput(v, t))); vecsort(Vec(v)) \\ Charles R Greathouse IV, Jul 19 2011 (Haskell) a046315 n = a046315_list !! (n-1) a046315_list = filter odd a001358_list  -- Reinhard Zumkeller, Jan 02 2014 CROSSREFS Odd members of A001358. A046388 is a subsequence. Sequence in context: A225438 A175076 A046337 * A046372 A025045 A107987 Adjacent sequences:  A046312 A046313 A046314 * A046316 A046317 A046318 KEYWORD nonn AUTHOR Patrick De Geest, Jun 15 1998 STATUS approved

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Last modified January 20 19:53 EST 2020. Contains 331096 sequences. (Running on oeis4.)