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 A077362 Largest n-digit prime whose external digits as well as internal digits form a prime, or 0 if no such number exists. 2
 0, 0, 977, 9677, 99377, 998717, 9998777, 99999617, 999999017, 9999996437, 99999997397, 999999997277, 9999999986477, 99999999993317, 999999999997337, 9999999999990797, 99999999999998837, 999999999999995717 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Conjecture: no entry is zero for n>2. Conjecture: each term after the first two terms ends with 7. - Harvey P. Dale, May 26 2018 LINKS Harvey P. Dale, Table of n, a(n) for n = 0..100 MATHEMATICA LastDigit[n_] := n - 10*Floor[n/10]; FirstDigit[n_] := Floor[n/(10^(Ceiling[Log[10, n]] - 1))]; MiddleDigits[n_] := Floor[(n - Floor[n/(10^(Ceiling[Log[10, n]] - 1))]*10^(Ceiling[Log[10, n]] - 1))/10]; IntExtPrimeTest2[n_] := TrueQ[(Boole[PrimeQ[FirstDigit[n]*10 + LastDigit[ n]]] + Boole[PrimeQ[MiddleDigits[n]]] + Boole[PrimeQ[n]]) == 3]; finder[digits_] := (maxj = 10^digits; For[j = maxj, IntExtPrimeTest2[j] == False, j-- ]; Print[j]); Do[finder[n], {n, 3, 25}] - Joshua Albert (jba138(AT)psu.edu), Feb 22 2006 eidQ[n_]:=Module[{idn=IntegerDigits[n]}, AllTrue[{FromDigits[Join[ {idn[[1]]}, {idn[[-1]]}]], FromDigits[Most[Rest[idn]]]}, PrimeQ]]; Join[ {0, 0}, Table[Module[{np=NextPrime[10^n-1, -1]}, While[ !eidQ[np], np = NextPrime[ np, -1]]; np], {n, 3, 18}]] (* The program uses the AllTrue function from Mathematica version 10 *) (* Harvey P. Dale, May 26 2018 *) CROSSREFS Cf. A069686, A077359, A077360, A077361. Sequence in context: A288917 A264130 A282405 * A077380 A063052 A231708 Adjacent sequences:  A077359 A077360 A077361 * A077363 A077364 A077365 KEYWORD base,nonn AUTHOR Amarnath Murthy, Nov 05 2002 EXTENSIONS Corrected and extended by Joshua Albert (jba138(AT)psu.edu), Feb 22 2006 STATUS approved

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Last modified December 7 05:14 EST 2019. Contains 329839 sequences. (Running on oeis4.)