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A101700 Numbers of the form 3*(10^n-3), where 10^n-3 is prime. 6
21, 291, 2991, 299999999999999991 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
a(5) = 3*(10^140-3) is 141 digits long and is too large to include.
If m is in this sequence then phi(m)=r(m), so this sequence is a subsequence of A069215. a(n)=3*(10^A089675(n)-3), so a(4)=3*(10^17-3), a(5)=3*(10^140-3), a(6)=3*(10^990-3), a(7)=3*(10^1887-3), a(8)=3*(10^3530-3), a(9)=3*(10^5996-3), a(10)=3*(10^13820-3), a(11)=3*(10^21873-3) & a(12)=3*(10 ^26045-3).
LINKS
FORMULA
a(n) = 3*(10^A089675(n) - 3).
EXAMPLE
299999999999999991 is in the sequence because (10^17-3) is prime and 3*(10^17-3)=299999999999999991.
MATHEMATICA
Do[If[PrimeQ[10^n-3], Print[3*(10^n-3)]], {n, 150}]
3#&/@Select[10^Range[20]-3, PrimeQ] (* Harvey P. Dale, Mar 23 2022 *)
CROSSREFS
Sequence in context: A086947 A230768 A230021 * A275359 A022291 A025944
KEYWORD
nonn
AUTHOR
Farideh Firoozbakht, Dec 31 2004
STATUS
approved

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Last modified May 30 03:30 EDT 2024. Contains 372957 sequences. (Running on oeis4.)