

A080440


a(1) = 13, a(n) = smallest prime obtained by inserting digits between every pair of digits of a(n1).


4




OFFSET

1,1


COMMENTS

Conjecture: Only one digit needs to be inserted between each pair of digits of a(n1) to get a(n); i.e., a(n) contains exactly 2n1 digits for n > 1.
The conjecture above is false: a(4)=100000963 has 9 digits instead of 2*41=7. A refined conjecture is: a(n) contains exactly 2^(n1) + 1 digits for all n>0. This follows trivially by induction from the initial above conjecture of only one digit needed between each pair, and the fact that we start with 13, a 2 digit number, and holds true at least till a(12).  Julio Cesar HernandezCastro, Jul 06 2011


LINKS

Julio Cesar HernandezCastro, Table of n, a(n) for n = 1..12


MATHEMATICA

a[n_] := Block[{d = IntegerDigits[n]}, k = Length[d]; While[k > 1, d = Insert[d, 0, k]; k ]; d = FromDigits[d]; e = d; k = 0; While[ !PrimeQ[e], k++; e = d + 10FromDigits[ IntegerDigits[k], 100]]; e]; NestList[a, 13, 6]


CROSSREFS

Cf. A080439, A080441, A080442, A080883  A080914.
Sequence in context: A240804 A100277 A087398 * A159352 A289859 A129762
Adjacent sequences: A080437 A080438 A080439 * A080441 A080442 A080443


KEYWORD

base,nonn


AUTHOR

Amarnath Murthy, Feb 22 2003


EXTENSIONS

Edited, corrected and extended by Robert G. Wilson v, Feb 22 2003


STATUS

approved



