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 A075243 Composite numbers requiring increasingly larger bases to become prime by base reversal. 2
 4, 6, 8, 9, 16, 27, 36, 78, 81, 102, 114, 144, 270, 420, 480, 750, 1848, 2382, 2940, 13860, 14490, 14520, 21840, 31860, 33660, 44580, 80850, 1228080, 4305210, 5326860, 6846840, 9796710, 9990750, 10720710, 14910630, 15129510, 15278250, 16785090, 17022390, 17608500 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The bases at which these entries occur are in A074901. See A075242. LINKS Amiram Eldar, Table of n, a(n) for n = 1..48 MATHEMATICA NextComposite[n_] := Block[{k = n + 1}, While[PrimeQ[k], k++ ]; k]; f[n_] := Block[{b = 2}, While[b < n && !PrimeQ[ FromDigits[ Reverse[ IntegerDigits[n, b]], b]], b++ ]; b]; If[b != n, b, 0]; b = -1; k = 4; Do[ While[c = f[k]; c <= b, k = NextComposite[k]]; b = c; Print[k], {n, 1, 31}] CROSSREFS Cf. A074901, A075242. Sequence in context: A095404 A073540 A123710 * A024370 A121719 A267509 Adjacent sequences:  A075240 A075241 A075242 * A075244 A075245 A075246 KEYWORD base,hard,nonn AUTHOR Robert G. Wilson v, Sep 09 2002 EXTENSIONS More terms from Amiram Eldar, Jun 04 2021 STATUS approved

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Last modified September 26 12:04 EDT 2021. Contains 347665 sequences. (Running on oeis4.)