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A160432 Primes of the form 3*10^(2*n) + 3*10^n + 1. 3
7, 331, 300030001, 3000000003000000001 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Primes of the form (x^3-y^3)/(x-y) with x = y+1 (which gives A002407) and also y=10^k for some k.

These prime numbers (differences of consecutive cubes: A002407), for k>0, have only three digits different from zero. The first is 3, the middle digit is 3 and the final digit is 1. The other 2(k-1) digits are value 0.

If k=6*i or k=6*i-1 the number is always divisible by 7. [Giacomo Fecondo, May 22 2010]

LINKS

Table of n, a(n) for n=1..4.

EXAMPLE

a(1) = 7 = (10^0+1)^3 -(10^0)^3 , 2^3-1^3.

a(2) = 331 =(10^1+1)^3 -(10^1)^3, 11^3-10^3.

a(3) = 300030001 = (10^4+1)^3 - (10^4)^3, 10001^3-10000^3.

a(1)= 3t(t+1)+1 with t=10^0; a(2)= 3t(t+1)+1 with t=10^1; a(3)= 3t(t+1)+1 with t=10^4.

For k=102 (k=6*17) the number (10^102+1)^3-(10^102)^3 is divisible by 7; for k=101 (k=6*17-1) the number (10^101+1)^3-(10^101)^3 is divisible by 7. [Giacomo Fecondo, May 22 2010]

MATHEMATICA

Select[Table[3*10^(2 n) + 3*10^n + 1, {n, 0, 1000}], PrimeQ] (* Vincenzo Librandi, Jan 28 2013 *)

PROG

(MAGMA) [a: n in [0..30] | IsPrime(a) where a is 3*10^(2*n) + 3*10^n + 1]; // Vincenzo Librandi, Jan 28 2013

(PARI) A160432(n, print_all=0, Start=0, Limit=9e9)={for(k=Start, Limit, ispseudoprime(p=3*100^k+3*10^k+1) & !(print_all & print1(p", ")) & !n-- & return(p))} \\ - M. F. Hasler, Jan 28 2013

CROSSREFS

Cf. A002407, A003215.

Sequence in context: A092588 A049686 A177019 * A324233 A009587 A281619

Adjacent sequences:  A160429 A160430 A160431 * A160433 A160434 A160435

KEYWORD

nonn

AUTHOR

Giacomo Fecondo, May 13 2009

EXTENSIONS

New name from Vincenzo Librandi, Jan 28 2013

STATUS

approved

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Last modified December 15 17:41 EST 2019. Contains 330000 sequences. (Running on oeis4.)