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 A113572 Least n-digit multiple of n such that the r-th digit is prime if r is a prime else it is composite. The location and value of the most significant digit is 1. 0 if no such number exists. 2
 1, 12, 123, 1224, 12245, 122424, 1224265, 12242424, 122424246, 0, 12242424447, 122424244428, 1224242444243, 12242424442458, 0, 1224242444242448, 12242424442424692, 122424244424244828, 1224242444242444367, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS a(5*t)=0, for all t>1. This is because the last digit of 5*t is always 0 or 5 yet we require this digit to be composite for t>1. There are no other zero terms below a(10000). Conjecture: No other term is zero. Because 0 is neither prime nor composite, it does not appear in any nonzero term. The digit 1 may appear only as the first digit of a term. LINKS Sean A. Irvine, Table of n, a(n) for n = 1..1000 EXAMPLE a(6) = 122424, the first digit is 1, the 2nd, 3rd and the 5th digits are primes and the 4th and 6th digit are composite. MATHEMATICA okQ[n_]:=Module[{d=IntegerDigits[n], ok, i=1}, ok=(d[[1]]==1); While[i

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Last modified May 17 06:30 EDT 2021. Contains 343965 sequences. (Running on oeis4.)