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A193227
Semiprimes p*q such that p+1 and q+1 are semiprimes.
2
9, 15, 25, 39, 65, 111, 169, 183, 185, 219, 305, 365, 471, 481, 579, 785, 793, 831, 939, 949, 965, 1191, 1263, 1369, 1371, 1385, 1565, 1623, 1839, 1983, 1985, 2019, 2041, 2105, 2199, 2257, 2271, 2285, 2509, 2631, 2701, 2705, 2991
OFFSET
1,1
COMMENTS
Numbers of the form A005383(i)*A005383(j) for i,j >= 1. - Altug Alkan, Mar 22 2018
LINKS
EXAMPLE
1371 is in the sequence because 1371 = 3 * 457, and 3 + 1 = 4 and 457 + 1 = 2 * 229 are semiprimes.
MAPLE
with(numtheory):for n from 2 to 3000 do: x:=factorset(n):y:=bigomega(n):z:=x[1]:zz:=n/z:if y=2 and type(z, prime)=true and type(zz, prime) = true and bigomega(z+1)=2 and bigomega(zz+1)=2 then printf(`%d, `, n): else fi:od:
# Alternate:
N:= 10000: # to get all terms <= N
P:= select(p -> isprime(p) and numtheory:-bigomega(p+1)=2, [$1..N/3]):
nP:= nops(P):
sort(select(`<=`, [seq(seq(P[i]*P[j], i=1..j), j=1..nP)], N)); # Robert Israel, Mar 22 2018
MATHEMATICA
Take[Sort[Times@@@Select[Flatten[Table[{Prime[p], Prime[q]}, {p, 2, 200}, {q, p}], 1], PrimeOmega[#[[1]] + 1] == 2 && PrimeOmega[#[[2]] + 1] == 2 &]], 45] (* Alonso del Arte, Jul 18 2011 *)
cQ[n_]:=Module[{fi=FactorInteger[n]}, Which[PrimeOmega[n]==2&&IntegerQ[Sqrt[ n]], PrimeOmega[ Sqrt[n]+1]==2, PrimeOmega[n] == 2, PrimeOmega[ 1+ fi[[All, 1]]] =={2, 2}, True, False]]; Select[Range[3000], cQ]
PROG
(PARI) list(lim)=my(v=List(), u=List(), t); forprime(p=3, lim\3, if(isprime((p+1)/2), listput(v, p))); for(i=1, #v, for(j=i, #v, t=v[i]*v[j]; if(t>lim, break); listput(u, t))); vecsort(Vec(u)) \\ Charles R Greathouse IV, Jul 18 2011
CROSSREFS
Subsequence of A001358.
Sequence in context: A342418 A251415 A109888 * A014003 A333788 A082549
KEYWORD
nonn
AUTHOR
Michel Lagneau, Jul 18 2011
STATUS
approved