|
| |
|
|
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 which are primes.
|
|
1
|
|
|
|
1, 2, 3, 5, 6, 7, 16, 17, 18, 104, 127, 213, 1143
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
|
OFFSET
|
0,2
|
|
|
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
|
Table of n, a(n) for n=0..12.
|
|
|
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}] (from Robert G. Wilson v Feb 10 2004)
|
|
|
PROG
|
(PARI) test7(n)= s=0; for(i=0, n, \ if(Mod(i, 5)==0, s=s+3*10^i, \ if(Mod(i, 5)==1, s=s+7*10^i, \ if(Mod(i, 5)==2, s=s+9*10^i, \ if(Mod(i, 5)==3, s=s+1*10^i, \ if(Mod(i, 5)==4, s=s+5*10^i, \ )))))); return(s); for(j=0, 1000, if(isprime(test7(j)), print(j+1), print1(".")))
|
|
|
CROSSREFS
|
Sequence in context: A103538 A144671 A073721 * A002229 A146747 A077674
Adjacent sequences: A090742 A090743 A090744 * A090746 A090747 A090748
|
|
|
KEYWORD
|
hard,nonn,base,more
|
|
|
AUTHOR
|
mohammed bouayoun (bouyao(AT)wanadoo.fr), Feb 03 2004
|
|
|
EXTENSIONS
|
Corrected and extended by Robert G. Wilson v, Feb 10 2004
|
|
|
STATUS
|
approved
|
| |
|
|
