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

1,1

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 A367176 A121719

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