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A153328
Numbers k such that (10^k-1)*120/99+1 is prime.
0
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.
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
Sequence in context: A047178 A132095 A332441 * A068588 A083333 A032359
KEYWORD
nonn,base,more
AUTHOR
Cino Hilliard, Dec 23 2008
STATUS
approved