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Numbers n such that spf(n)+2 = spf(n-2), where spf=A020639 (smallest prime factor).
2

%I #10 Oct 15 2015 10:34:06

%S 27,57,87,117,147,177,205,207,237,267,297,327,357,387,415,417,447,477,

%T 507,537,567,597,625,627,657,687,717,747,777,807,835,837,867,897,927,

%U 957,987,1017,1045,1047,1077,1107,1137,1167,1197,1227,1255,1257,1287

%N Numbers n such that spf(n)+2 = spf(n-2), where spf=A020639 (smallest prime factor).

%C (spf(n),spf(n-2)) are twin prime pairs.

%H Harvey P. Dale, <a href="/A080864/b080864.txt">Table of n, a(n) for n = 1..1000</a>

%e 1921=17*113, 1921-2=1919=19*101 and (17,19)=(A001359(4), A006512(4)), therefore 1921 is a term: a(74)=1921, A080863(74)=1919.

%t Flatten[Position[Partition[Array[FactorInteger[#][[1,1]]&,1300],3,1],_?(#[[1]]==#[[3]]+2&),{1},Heads->False]]+2 (* _Harvey P. Dale_, Oct 15 2015 *)

%Y Cf. A080863(n)=a(n)-2, A001359, A006512.

%K nonn

%O 1,1

%A _Reinhard Zumkeller_, Feb 21 2003