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 A089706 Largest n-digit prime in which the unit place digit is 1 and the k-th most significant digit is prime (2,3,5,7) if k is prime else is composite (4,6,8,9,0). 1
 71, 751, 9721, 79531, 978521, 7979771, 97979771, 997979371, 9997979371, 79997979521, 979997978551, 7979997979571, 97979997979771, 997979997978571, 9997979997978571, 79997979997979731, 979997979997978531 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,1 LINKS EXAMPLE a(6) = 420221. MAPLE with(combinat, cartprod): ds:=proc(s) local j, l: l:=nops(s): RETURN(add(s[j]*10^(l-j), j=1..l)):end: p:=[7, 5, 3, 2]:c:=[9, 8, 6, 4, 0]: cf:=[9, 8, 6, 4]: ctpr:=proc(n, s) local m, T, a: a:=s: m:=1: T:=cartprod([seq(piecewise(isprime(n-i+2), p, i=2, cf, c), i=2..n), [1]]): while not T[finished] do m:=ds(T[nextvalue]()): if isprime(m) and not member(m, a) then a:=[op(a), m]: RETURN(a) fi od: end: a:=[]: for n from 2 to 22 do a:=ctpr(n, a) od: op(a); # C. Ronaldo CROSSREFS Cf. A089704, A089705. Sequence in context: A201039 A142674 A112750 * A220623 A173806 A253683 Adjacent sequences:  A089703 A089704 A089705 * A089707 A089708 A089709 KEYWORD base,nonn AUTHOR Amarnath Murthy, Nov 10 2003 EXTENSIONS More terms from C. Ronaldo (aga_new_ac(AT)hotmail.com), Dec 25 2004 STATUS approved

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Last modified September 22 17:39 EDT 2021. Contains 347607 sequences. (Running on oeis4.)