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A138840
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Concatenation of initial and final digits of n-th prime.
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8
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22, 33, 55, 77, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 11, 13, 17, 19, 13, 17, 11, 17, 19, 19, 11, 17, 13, 17, 13, 19, 11, 11, 13, 17, 19, 21, 23, 27, 29, 23, 29, 21, 21, 27, 23, 29, 21, 27, 21, 23, 23, 37, 31, 33, 37, 31, 37, 37
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listen;
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OFFSET
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1,1
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COMMENTS
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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
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LINKS
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MAPLE
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a:= n-> (p-> parse(cat(p[1], p[-1])))(""||(ithprime(n))):
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MATHEMATICA
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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 *)
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PROG
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(PARI) a(n) = my(d=digits(prime(n))); fromdigits(concat(d[1], d[#d])); \\ Michel Marcus, Mar 23 2018
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CROSSREFS
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Cf. A137589 (same except for first four terms).
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KEYWORD
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AUTHOR
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STATUS
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approved
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