| Displaying 1-4 of 4 results found.
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5, 7, 43, 769, 1123, 52631, 1428571, 2127659, 14925373, 769230769, 5882352941, 6369426751, 588235294117, 319488817891, 9345794392523, 24449877750611, 662251655629139, 1293661060802069, 67114093959731543, 244498777506112469
COMMENTS
Conjecture: No term is zero.
PROG
(PARI) A090520(n)={ local(p, tenn) ; p=2 ; tenn=10^n ; while(tenn/p>=2, if( isprime(floor(tenn/p)), return(floor(tenn/p)) ) ; p=nextprime(p+1) ; ) ; return(0) ; } { for(n=1, 40, print1( A090520(n), ", ") ; ) } - R. J. Mathar, Nov 19 2006
5, 11, 83, 769, 3571, 52631, 1428571, 3703703, 83333333, 769230769, 5882352941, 13513513513, 588235294117, 7142857142857, 12195121951219, 151515151515151, 2777777777777777, 22727272727272727, 1111111111111111111
COMMENTS
Conjecture: No term is zero.
PROG
(PARI) A090518(n)={ local(k, tenn) ; k=1 ; tenn=10^n ; while(1, if( isprime(floor(tenn/k)), return(floor(tenn/k)) ) ; k++ ; ) ; } { for(n=1, 40, print1( A090518(n), ", ") ; ) } - R. J. Mathar, Nov 19 2006
Least k such that floor[(10^n)/k] is prime.
+10
3
2, 9, 12, 13, 28, 19, 7, 27, 12, 13, 17, 74, 17, 14, 82, 66, 36, 44, 9, 36, 21, 13, 9, 90, 7, 19, 149, 51, 321, 35, 12, 14, 140, 13, 28, 42, 34, 36, 153, 155, 133, 46, 73, 106, 162, 109, 122, 42, 62, 422, 29, 231, 38, 34, 340, 295, 151, 197, 94, 19, 17, 83, 131, 66, 36
EXAMPLE
a(1) = 2 as 10/2 = 5 is a prime. a(3) = 12 as floor[1000/12] = 83 is prime.
MATHEMATICA
lkp[n_]:=Module[{k=2, c=10^n}, While[!PrimeQ[Floor[c/k]], k++]; k]; Array[ lkp, 70] (* Harvey P. Dale, Jul 09 2018 *)
EXTENSIONS
Corrected and extended by Mohammed Bouayoun (bouyao(AT)wanadoo.fr), Jan 28 2004
Numbers k such that floor(10^k/7) is prime.
+10
1
COMMENTS
Numbers k such that A090519(k) = 7. All terms == 1 (mod 6).
Numbers k such that (10^k-3)/7 is prime.
EXAMPLE
a(1) = 7 is a term because floor(10^7/7) = 1428571 is prime.
MAPLE
select(n -> isprime(floor(10^n/7)), [seq(i, i=1..1000, 6)]);
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