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 A084632 Semiprimes such that when they are concatenated with their 10's complement, which also must be a semiprime, the result is a prime. 1

%I #9 Jan 02 2019 14:11:58

%S 49,91,169,237,319,321,329,377,411,417,473,529,553,583,597,629,697,

%T 713,763,779,791,817,913,923,949,1203,1257,1273,1339,1347,1379,1389,

%U 1497,1501,1509,1529,1589,1633,1739,1803,1841,1967,1969,2019,2103,2173,2219

%N Semiprimes such that when they are concatenated with their 10's complement, which also must be a semiprime, the result is a prime.

%H Harvey P. Dale, <a href="/A084632/b084632.txt">Table of n, a(n) for n = 1..1000</a>

%e a(3)=169 because 169 is a semiprime and 1000-169 = 831 is a semiprime, while 169831 is a prime.

%t sctQ[n_]:=Module[{tc=10^IntegerLength[n]-n,j},j=n*10^IntegerLength[ tc]+ tc; PrimeOmega[tc]==PrimeOmega[n]==2&&PrimeQ[j]]; Select[Range[ 2300], sctQ] (* _Harvey P. Dale_, Jan 02 2019 *)

%Y Cf. A001358.

%K nonn,base

%O 1,1

%A _Jason Earls_, Jun 28 2003

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Last modified June 9 13:49 EDT 2023. Contains 363180 sequences. (Running on oeis4.)