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A089749 a(n) = smallest m > a(n-1) such that concatenation nm is a prime. 2

%I

%S 1,3,7,19,21,31,33,39,41,49,51,59,61,71,79,93,107,119,121,123,139,147,

%T 159,169,171,177,179,181,191,197,219,233,247,253,257,263,273,281,293,

%U 343,351,359,391,417,427,439,441,449,451,459,461,489,503,517,529,531

%N a(n) = smallest m > a(n-1) such that concatenation nm is a prime.

%C Up to a(10^6), a(n) =~ 100000 + 24n + 10^(-6*n), which is almost a straight line. - _Robert G. Wilson v_

%e a(5) = 21 because the first prime after a(4) = 419 beginning with 5 is 521.

%t a[1] = 1; a[1] = 1; a[n_] := a[n] = Block[{k = IntegerDigits[n], l = a[n - 1] + 2}, While[ !PrimeQ[ FromDigits[ Join[k, IntegerDigits[l]]]], l += 2]; l]; Table[ a[n], {n, 1, 55}]

%K base,nonn

%O 1,2

%A _Amarnath Murthy_, Nov 20 2003

%E Corrected and extended by _Robert G. Wilson v_, Dec 05 2003

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Last modified November 22 00:32 EST 2019. Contains 329383 sequences. (Running on oeis4.)