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A117430 Integer k such that 5^n + k = A117429(n).

%I A117430 #21 Jul 23 2020 05:02:48
%S A117430 3,-1,0,-2,1,2,-2,-2,-2,-3,2,2,-2,-4,4,2,-8,-6,-2,-3,-2,-2,4,2,-6,-2,
%T A117430 4,2,-3,17,9,-4,-8,-6,12,14,-2,-6,-8,-2,-6,24,-2,14,-6,-4,-18,-6,-3,
%U A117430 -6,16,-10,16,-12,12,-2,16,6,16,-12,-2,-6,12,-12,-8,-19,-6,6,24,-16,4,2,16,-4,-8,-4,16
%N A117430 Integer k such that 5^n + k = A117429(n).
%C A117430 (+/-) distance from 5^n to the nearest semiprime.
%C A117430 a(0)=3 and a(1)=-1 are the only terms == 3 (mod 4), as 5^n + 3 is divisible by 4. - _Robert Israel_, May 03 2018
%H A117430 Robert Israel, <a href="/A117430/b117430.txt">Table of n, a(n) for n = 0..111</a>
%F A117430 a(n) = Integer k such that 5^n + k = A117429(n). a(n) = A117429(n) - 5^n. a(n) = Min{k such that A001358(i) + k = 5^n}.
%e A117430 a(0) = 3 because 5^0 + 3 = 4 = A001358(1) and no semiprime is closer to 5^0.
%e A117430 a(1) = -1 because 5^1 - 1 = 4 = A001358(1) and no semiprime is closer to 5^1.
%e A117430 a(2) = 0 because 5^2 + 0 = 25 = A001358(9), no semiprime is closer to 5^2 [this is the only 0 element].
%e A117430 a(3) = -2 because 5^3 - 2 = 123 = 3 * 41 = A001358(42), no semiprime is closer.
%e A117430 a(4) = 1 because 5^4 + 1 = 626 = 2 * 313, no semiprime is closer.
%e A117430 a(5) = 2 because 5^5 + 2 = 3127 = 53 * 59, no semiprime is closer.
%p A117430 nsp:= proc(n) uses numtheory; local k;
%p A117430   if bigomega(n)=2 then return n fi;
%p A117430   for k from 1 do
%p A117430     if n-k > 0 and bigomega(n-k)=2 then return n-k fi;
%p A117430     if bigomega(n+k)=2 then return n+k fi
%p A117430   od
%p A117430 end proc:
%p A117430 seq(nsp(5^n)-5^n, n=0..30); # _Robert Israel_, May 03 2018
%t A117430 nsp[n_] := Module[{k}, If[PrimeOmega[n] == 2, Return[n]]; For[k = 1, True, k++, If[n-k > 0 && PrimeOmega[n-k] == 2, Return[n-k]]; If[PrimeOmega[n+k] == 2, Return[n+k]]]];
%t A117430 a[n_] := a[n] = nsp[5^n] - 5^n;
%t A117430 Table[Print[n, " ", a[n]]; a[n], {n, 0, 76}] (* _Jean-François Alcover_, Jul 23 2020, after Maple *)
%Y A117430 Cf. A000079, A001358, A117387, A117405, A117406, A117416, A117429.
%K A117430 sign
%O A117430 0,1
%A A117430 _Jonathan Vos Post_, Mar 14 2006
%E A117430 More terms from _Robert Israel_, May 03 2018