A075243 Composite numbers requiring increasingly larger bases to become prime by base reversal.

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

Offset: 1

Author

Robert G. Wilson v, Sep 09 2002

Keywords

• base
• hard
• nonn

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

Crossrefs

Cf. A074901, A075242.

Programs

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}]

Extensions

More terms from Amiram Eldar, Jun 04 2021