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%I #41 Nov 23 2023 19:49:37
%S 22,33,55,77,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,
%T 89,97,11,13,17,19,13,17,11,17,19,19,11,17,13,17,13,19,11,11,13,17,19,
%U 21,23,27,29,23,29,21,21,27,23,29,21,27,21,23,23,37,31,33,37,31,37,37
%N Concatenation of initial and final digits of n-th prime.
%C There are only 38 distinct terms in this sequence, all of them odd with the exception of 22. 55 is the only term divisible by 5. 22 and 55 each appear only once. The other terms, each of which appears multiple times, are the odd two-digit numbers not divisible by 5. - _Harvey P. Dale_, May 15 2012
%C a(n) is the concatenation of A077648(n) and A007652(n), hence all terms of this sequence have two digits in the same way as A073729. - _Omar E. Pol_, Mar 23 2018
%H Harvey P. Dale, <a href="/A138840/b138840.txt">Table of n, a(n) for n = 1..1000</a>
%H Omar E. Pol, <a href="http://www.polprimos.com">Determinacion geometrica de los numeros primos y perfectos</a>.
%p a:= n-> (p-> parse(cat(p[1], p[-1])))(""||(ithprime(n))):
%p seq(a(n), n=1..92); # _Alois P. Heinz_, Nov 23 2023
%t cifd[n_]:=Module[{il=IntegerLength[n],idn=IntegerDigits[n]},Which[ il==1, 10n+n, il==2,n,il>2,FromDigits[Join[{First[idn],Last[idn]}]]]]; cifd/@ Prime[ Range[70]] (* _Harvey P. Dale_, May 15 2012 *)
%o (PARI) a(n) = my(d=digits(prime(n))); fromdigits(concat(d[1], d[#d])); \\ _Michel Marcus_, Mar 23 2018
%Y Cf. A000040, A007652, A073729, A077648, A138841, A138842, A138843, A138844.
%Y Cf. A137589 (same except for first four terms).
%K base,easy,nonn,look
%O 1,1
%A _Omar E. Pol_, Apr 01 2008
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