A153328 Numbers k such that (10^k-1)*120/99+1 is prime.

6, 10, 42, 138, 626, 1398, 1596, 1978, 7808, 14058

Offset

1,1

Comments

Also 2*A056803 which I took the liberty of using to create the last 2 entries.

These numbers are always even. If n is odd, then 10^n-1 produces a number with an odd number of 9's which 99 does not divide. a(6), a(10), a(42) are 1212121, 12121212121, 1212121212121212121212121212121212121212121 which can be found in A092696. Also, the formula produce palindromic numbers.

Links

Table of n, a(n) for n=1..10.

Prog

(PARI) /* n=number of values to test; r=repeat digits, e.g., 12, 121, 177, 1234; d = last digit appended to the end */
repr(n, r, d) = ln=length(Str(r)); for(x=0, n, y=(10^(ln*x)-1)*10*r/ (10^ln-1)+1; if(ispseudoprime(y), print1(ln*x", ")))

Crossrefs

Cf. A092696, A056803.

Sequence in context: A047178 A132095 A332441 * A068588 A083333 A032359

Adjacent sequences: A153325 A153326 A153327 * A153329 A153330 A153331

Keyword

nonn,base,more

Author

Cino Hilliard, Dec 23 2008

Status

approved