The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation. Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A252491 a(n) = (10^(n^2) - 1)/(10^n - 1). 4
 1, 101, 1001001, 1000100010001, 100001000010000100001, 1000001000001000001000001000001, 1000000100000010000001000000100000010000001, 100000001000000010000000100000001000000010000000100000001, 1000000001000000001000000001000000001000000001000000001000000001 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS When written in base 10, the terms consist of n digits '1' separated by strings of n-1 digits '0'. This sequence is relevant for counterexamples to a conjecture in A086766: If p is prime and a(p) is not prime, then A086766(10^(p-1)) = 0. a(n) is the product of A019328(d) for all d that divide n^2 but not n. - Robert Israel, Jan 08 2015 If a(n) is a prime then n is a prime. What is the smallest prime term greater than 101 in this sequence? - Farideh Firoozbakht, Jan 08 2015 According to what precedes, a(n) is prime iff A019328(d) is prime, where d is the only divisor of n^2 which is not a divisor of n, i.e., iff n is a prime and n^2 is in A138940. No such term is known, except for n=2. - M. F. Hasler, Jan 09 2015 LINKS MAPLE seq((10^(n^2)-1)/(10^n-1), n=1..20); # Robert Israel, Jan 08 2015 PROG (PARI) A252491(n)=(10^(n^2)-1)\(10^n-1) CROSSREFS Sequence in context: A180053 A138721 A068659 * A015078 A031982 A015041 Adjacent sequences:  A252488 A252489 A252490 * A252492 A252493 A252494 KEYWORD nonn,easy AUTHOR M. F. Hasler, Jan 08 2015 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
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified April 8 21:44 EDT 2020. Contains 333329 sequences. (Running on oeis4.)