A235108 Semiprimes with digits in strictly decreasing order.
4, 6, 9, 10, 21, 51, 62, 65, 74, 82, 85, 86, 87, 91, 93, 94, 95, 321, 542, 543, 721, 731, 753, 763, 831, 841, 842, 843, 851, 862, 865, 871, 921, 943, 951, 961, 965, 973, 974, 982, 985, 4321, 5321, 6431, 6541, 6542, 7421, 7431, 7531, 7543, 7631, 7642, 7651
Offset: 1
Links
- Donovan Johnson, Table of n, a(n) for n = 1..159 (complete sequence)
Programs
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Maple
b:= proc(n) n, seq(b(10*n+j), j=irem(n, 10)-1..0, -1) end: select(numtheory[bigomega]=2, {seq(b(n), n=1..9)})[]; # Alois P. Heinz, Apr 13 2025 -
Mathematica
Select[Range[10000],PrimeOmega[#]==2&&Max[Differences[IntegerDigits[#]]]<0&] (* Harvey P. Dale, Aug 18 2024 *)
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PARI
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 isdecr(a) = {my(b=-1); until(a==0, if(a%10<=b, return(0)); b=a%10; a\=10); return(1)} u=list(50000); s=[]; for(n=1, #u, if(isdecr(u[n]), s=concat(s, u[n]))); s