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A173082 Semiprimes q such that q^2+2 and q^2-2 are also semiprimes. 5
6, 51, 65, 115, 133, 141, 159, 187, 201, 209, 213, 287, 291, 295, 327, 339, 361, 407, 411, 413, 471, 493, 511, 519, 537, 559, 579, 597, 633, 649, 687, 695, 723, 799, 813, 831, 835, 871, 917, 939, 1007, 1041, 1047, 1079, 1135, 1167, 1189, 1195, 1199, 1227 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..50.

FORMULA

{A001358(j): A074985(j)-2 in A001358 and A074985(j)+2 in A001358}. [R. J. Mathar, Mar 11 2010]

EXAMPLE

6^2-2=34 = 2*17 and 6^2+2=38 = 2*19 are semiprimes derived from the semiprime q=6, so q=6 is in the sequence.

MATHEMATICA

f[n_]:=Last/@FactorInteger[n]=={1, 1}||Last/@FactorInteger[n]=={2}; lst={}; Do[If[f[n], a=n^2-2; b=n^2+2; If[f[a]&&f[b], AppendTo[lst, n]]], {n, 8!}]; lst

CROSSREFS

Cf. A001358

Sequence in context: A043084 A113653 A292818 * A133395 A050916 A011790

Adjacent sequences:  A173079 A173080 A173081 * A173083 A173084 A173085

KEYWORD

nonn

AUTHOR

Vladimir Joseph Stephan Orlovsky, Feb 09 2010

EXTENSIONS

Definition rephrased - R. J. Mathar, Mar 11 2010

STATUS

approved

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Last modified September 20 16:48 EDT 2019. Contains 327242 sequences. (Running on oeis4.)