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A106557 Largest number that can be obtained by concatenating the two factors of the n-th semiprime.

%I A106557 #12 Jan 08 2020 20:40:48
%S A106557 22,32,33,52,72,53,73,211,55,213,311,217,75,219,313,232,77,317,511,
%T A106557 319,292,312,513,323,372,711,412,517,432,329,713,331,472,519,532,373,
%U A106557 523,592,717,1111,612,413,433,719,672,473,712,1311,529,732,531,792,533
%N A106557 Largest number that can be obtained by concatenating the two factors of the n-th semiprime.
%H A106557 Andrew Howroyd, <a href="/A106557/b106557.txt">Table of n, a(n) for n = 1..1000</a>
%F A106557 a(n) = A084797(A001358(n)). - _Andrew Howroyd_, Jan 08 2020
%e A106557 First semiprime is 4; 4 is 2*2 -> 22.
%e A106557 Second semiprime is 6; 6 is 3*2 -> 32 (and not 23).
%e A106557 ...
%e A106557 Eighth semiprime is 22; 22 is 2*11 -> 211 (and not 112).
%o A106557 (PARI) \\ here cd(x,y) returns base 10 concatenation.
%o A106557 cd(v1, v2)={10^(logint(v2,10) + 1)*v1 + v2}
%o A106557 seq(n)={my(v=vector(n), k=0); for(i=1, #v, k++; while(2<>bigomega(k), k++); my(f=factor(k)[,1]); v[i] = if(#f==1, cd(f[1], f[1]), max(cd(f[1], f[2]), cd(f[2], f[1])))); v} \\ _Andrew Howroyd_, Jan 08 2020
%Y A106557 Cf. A001358, A084797, A106556.
%K A106557 base,easy,nonn
%O A106557 1,1
%A A106557 _Eric Angelini_, May 09 2005
%E A106557 Edited by _N. J. A. Sloane_, Apr 14 2008
%E A106557 Terms a(22) and beyond from _Andrew Howroyd_, Jan 08 2020