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A235108 Semiprimes with digits in strictly decreasing order.

%I A235108 #19 Apr 13 2025 20:49:59
%S A235108 4,6,9,10,21,51,62,65,74,82,85,86,87,91,93,94,95,321,542,543,721,731,
%T A235108 753,763,831,841,842,843,851,862,865,871,921,943,951,961,965,973,974,
%U A235108 982,985,4321,5321,6431,6541,6542,7421,7431,7531,7543,7631,7642,7651
%N A235108 Semiprimes with digits in strictly decreasing order.
%H A235108 Donovan Johnson, <a href="/A235108/b235108.txt">Table of n, a(n) for n = 1..159</a> (complete sequence)
%p A235108 b:= proc(n) n, seq(b(10*n+j), j=irem(n, 10)-1..0, -1) end:
%p A235108 select(numtheory[bigomega]=2, {seq(b(n), n=1..9)})[];  # _Alois P. Heinz_, Apr 13 2025
%t A235108 Select[Range[10000],PrimeOmega[#]==2&&Max[Differences[IntegerDigits[#]]]<0&] (* _Harvey P. Dale_, Aug 18 2024 *)
%o A235108 (PARI)
%o A235108 list(lim)=my(v=List(), t); forprime(p=2, sqrt(lim), t=p; forprime(q=p, lim\t, listput(v, t*q))); vecsort(Vec(v)) \\ From A001358
%o A235108 isdecr(a) = {my(b=-1); until(a==0, if(a%10<=b, return(0)); b=a%10; a\=10); return(1)}
%o A235108 u=list(50000); s=[]; for(n=1, #u, if(isdecr(u[n]), s=concat(s, u[n]))); s
%Y A235108 Cf. A001358, A108635, A108634, A135355.
%K A235108 nonn,base,fini,full
%O A235108 1,1
%A A235108 _Colin Barker_, Jan 03 2014