login
This site is supported by donations to The OEIS Foundation.

 

Logo

Annual Appeal: Please make a donation (tax deductible in USA) to keep the OEIS running. Over 5000 articles have referenced us, often saying "we discovered this result with the help of the OEIS".

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
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. 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}] (* 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy .

Last modified December 11 08:51 EST 2016. Contains 279052 sequences.