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A090744 Consider numbers of the form ...53197531975319753, whose digits read from the right are 3,5,7,9,1,3,5,7,9,1,3,... Sequence gives lengths of these numbers that are primes. 1
1, 2, 5, 26, 1932 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

LINKS

Table of n, a(n) for n=0..4.

EXAMPLE

a(1)=1 because 3 is prime and 3 has 1 digit.

a(2)=2 because 53 is prime and 53 has 2 digits.

a(3)=5 because 19753 is prime and 19753 has 5 digits.

MATHEMATICA

s = 0; Do[s = s + 10^n*Switch[ Mod[n, 5], 4, 1, 0, 3, 1, 5, 2, 7, 3, 9]; If[ PrimeQ[s], Print[n + 1]], {n, 0, 3000}] (* Robert G. Wilson v, Feb 19 2004 *)

PROG

(PARI) test3(n)= s=0; for(i=0, n, \ if(Mod(i, 5)==0, s=s+3*10^i, \ if(Mod(i, 5)==1, s=s+5*10^i, \ if(Mod(i, 5)==2, s=s+7*10^i, \ if(Mod(i, 5)==3, s=s+9*10^i, \ if(Mod(i, 5)==4, s=s+1*10^i, \ )))))); return(s); for(j=0, 1000, if(isprime(test3(j)), print(j+1), print1(".")))

CROSSREFS

Sequence in context: A023362 A138613 A299104 * A041571 A042259 A100105

Adjacent sequences:  A090741 A090742 A090743 * A090745 A090746 A090747

KEYWORD

hard,nonn,base,more

AUTHOR

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

EXTENSIONS

1932 from Robert G. Wilson v, Feb 10 2004

STATUS

approved

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Last modified February 23 04:30 EST 2019. Contains 320411 sequences. (Running on oeis4.)