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A080442 a(1) = 19, a(n) is the smallest prime obtained by inserting digits between every pair of digits of a(n-1). 4
19, 109, 10009, 100000039, 10000000000002359, 100000000000000000000000002031519, 10000000000000000000000000000000000000000000000000002000301050179 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Conjecture: Only one digit needs to be inserted between each pair of digit 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(5) = 10000000000002359 has 17 digits instead of 2*5 - 1 = 9. A refined conjecture is: a(n) contains exactly 2^(n-1) + 1 digits for all n > 0. This follows trivially from the initial conjecture of only one digit needed between each pair, and the fact that we start with 19, a 2-digit number, and holds true at least till a(12). - Julio Cesar Hernandez-Castro, Jul 07 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, 19, 6]
CROSSREFS
Sequence in context: A281170 A158715 A186105 * A033655 A245753 A164604
KEYWORD
base,nonn
AUTHOR
Amarnath Murthy, Feb 22 2003
EXTENSIONS
Edited and extended by Robert G. Wilson v, Feb 22 2003
STATUS
approved

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Last modified March 28 18:04 EDT 2024. Contains 371254 sequences. (Running on oeis4.)