

A090745


Consider numbers of the form ...7531975319753197, whose digits read from the right are 7,9,1,3,5,7,9,1,3,5,7,... Sequence gives lengths of these numbers that are primes.


2



1, 2, 3, 5, 6, 7, 16, 17, 18, 104, 127, 213, 1143, 4654, 4897, 17046, 33424
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OFFSET

1,2


COMMENTS

Numbers n such that 53197 * (10^(5*m)1)/99999 + (53197 mod 10^(n mod 5))*10^(5*m) is prime, where m = floor(n/5).  Max Alekseyev, Nov 11 2012


LINKS



EXAMPLE

a(1) = 1 because 7 is prime and 7 has 1 digits.
a(2) = 2 because 97 is prime and 97 has 2 digits.
a(3) = 3 because 197 is prime and 197 has 3 digits.


MATHEMATICA

s = 0; Do[s = s + 10^n*Switch[ Mod[n, 5], 2, 1, 3, 3, 4, 5, 0, 7, 1, 9]; If[ PrimeQ[s], Print[n + 1]], {n, 0, 3000}] (* Robert G. Wilson v, Feb 10 2004 *)
Position[Table[FromDigits[PadLeft[{}, n, {5, 3, 1, 9, 7}]], {n, 1200}], _?PrimeQ]// Flatten (* Harvey P. Dale, Mar 30 2019 *)


PROG

(PARI) test7(n)= s=0; for(i=0, n, if(Mod(i, 5)==0, s=s+7*10^i, if(Mod(i, 5)==1, s=s+9*10^i, if(Mod(i, 5)==2, s=s+1*10^i, if(Mod(i, 5)==3, s=s+3*10^i, if(Mod(i, 5)==4, s=s+5*10^i, )))))); return(s);


CROSSREFS



KEYWORD

hard,nonn,base,more,less


AUTHOR

mohammed bouayoun (bouyao(AT)wanadoo.fr), Feb 03 2004


EXTENSIONS



STATUS

approved



