OFFSET
1,1
COMMENTS
Conjecture: Only one digit needs to be inserted between each pair of digits of a(n-1) to get a(n); i.e., a(n) contains exactly 2n-1 digits for n > 1.
The conjecture above is false: a(4) = 100000963 has 9 digits instead of 2*4 - 1 = 7. A refined conjecture is: a(n) contains exactly 2^(n-1) + 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 Hernandez-Castro, Jul 06 2011
LINKS
Julio Cesar Hernandez-Castro, 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
KEYWORD
base,nonn
AUTHOR
Amarnath Murthy, Feb 22 2003
EXTENSIONS
Edited, corrected and extended by Robert G. Wilson v, Feb 22 2003
STATUS
approved