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%I #2 Mar 30 2012 17:30:50
%S 2,3,15,8,109,9,119,16,27,70,2197,36,1265,158,213,178,4205,126,14189,
%T 260,273,304,4865,120,1295,78,81,532,44323,150,47317,952,771,102,
%U 16705,492,6209,114,1209,2020,132743,294,22945,2834,2721,2276,66455,144
%N Least number requiring the base n to produce a prime by base reversal.
%C Question, Is every base necessary to convert the natural numbers into primes?
%e a(1) = 2 because two = 11 in unary (A000042) and its reversal 11 = 2. a(2) = 3 because three = 11 in base 2 (A007088) and its reversal 11 in base 2 = 3. a(3) = 15 because fifteen = 120 in base 3 (A007089) and its reversal 21 in base 3 = 7. a(4) = 8 -> 2. a(7) = 119 because 119 base 7 = 230 in base 7 (A007093) and its reversal 32 base 7 = 161.
%t f[n_] := Block[{b = 2}, While[b < n && !PrimeQ[ FromDigits[ Reverse[ IntegerDigits[n, b]], b]], b++ ]; If[b != n, b, 0]]; a = Table[0, {70}]; Do[b = f[n]; If[b < 76 && a[[b]] == 0, a[[b]] = n], {n, 2, 133000}]
%Y Cf. A075241 & A075242.
%K base,easy,nonn
%O 1,1
%A _Robert G. Wilson v_, Sep 09 2002
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