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 A108873 Numbers n whose base 3 representations, interpreted as base 10 integers, are semiprimes. 6
 3, 7, 8, 13, 16, 17, 19, 20, 25, 31, 37, 40, 43, 47, 49, 61, 64, 65, 71, 73, 82, 88, 89, 92, 97, 100, 101, 106, 110, 115, 118, 121, 127, 136, 142, 143, 155, 179, 184, 187, 188, 191, 209, 232, 235, 244, 250, 254, 259, 262, 263, 265, 269, 274, 281, 289, 299, 314, 319 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS EXAMPLE a(1) = 3 because 3 (base 10) = 10 (base 3) and 10 base 10 = 2 * 5. a(2) = 7 because 7 (base 10) = 21 (base 3) and 21 base 10 = 3 * 7. a(4) = 13 because 13 (base 10) = 111 (base 3) and 111 base 10 = 3 * 37. a(12) = 40 because 40 (base 10) = 1111 (base 3) and 1111 base 10 = 11 * 101. a(21) = 82 because 82 (base 10) = 10001 (base 3) and 10001 base 10 = 73 * 137. a(26) = 100 because 100 (base 10) = 10201 (base 3) and 10201 base 10 = 101^2. MAPLE with(numtheory): a:=proc(n) local nn, nnn: nn:=convert(n, base, 3): nnn:=add(nn[j]*10^(j-1), j=1..nops(nn)): if bigomega(nnn)=2 then n else fi end: seq(a(n), n=1..350); # Emeric Deutsch, Jul 16 2005 MATHEMATICA Select[Range[319], Plus @@ Last /@ FactorInteger[FromDigits[IntegerDigits[ #, 3]]] == 2 &] (* Ray Chandler, Sep 21 2005*) CROSSREFS Cf. A001358, A007089. Cf. A110602 (base 4), A110603 (base 5), A110604 (base 6), A110605 (base 7) , A110606 (base 8), A110607 (base 9). Sequence in context: A132017 A244592 A010342 * A002312 A252496 A279517 Adjacent sequences:  A108870 A108871 A108872 * A108874 A108875 A108876 KEYWORD base,easy,nonn AUTHOR Jonathan Vos Post, Jul 13 2005 EXTENSIONS More terms from Emeric Deutsch, Jul 16 2005 STATUS approved

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Last modified June 15 13:25 EDT 2021. Contains 345048 sequences. (Running on oeis4.)