OFFSET
1,1
COMMENTS
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..1000
D. Broadhurst (in reply to James Merickel), Re: Sphenic chain by concatenation of factors, "primenumbers" group, June 16, 2011.
James Merickel, David Broadhurst, Kevin Acres, Sphenic tree by factor concatenation on 114, digest of 16 messages in primenumbers Yahoo group, Jun 11, 2011 - Jun 23, 2011.
EXAMPLE
26=2*13 is in the sequence because 213 =3*71 is a product of two distinct primes.
MATHEMATICA
tdpQ[n_]:=Module[{fi=FactorInteger[n][[All, 1]]}, AnyTrue[ {fi[[1]]* 10^IntegerLength[fi[[2]]]+ fi[[2]], fi[[2]]*10^IntegerLength[ fi[[1]]]+ fi[[1]]}, PrimeNu[#]==PrimeOmega[#]==2&]]; Select[Range[ 500], PrimeOmega[#] == PrimeNu[#]==2&&tdpQ[#]&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Jul 20 2018 *)
PROG
CROSSREFS
KEYWORD
nonn,base
AUTHOR
M. F. Hasler, Jun 19 2011
STATUS
approved