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 A137589 a(n) is the integer that results after deletion of all digits of n-th prime, except the initial digit and the final digit. 2
 2, 3, 5, 7, 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, 39, 33 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The plot of this sequence shows number of primes on the x-axis and the split of primes into 9 groups according to their first digit on the y-axis. The plot of a(n+1)/a(n) oscillates around 1 with decreasing amplitude. Log-periodic growth is seen on the plot of partial sums b(n)= Sum_(i=1..n) a(i). LINKS David A. Corneth, Table of n, a(n) for n = 1..10000 FORMULA a(n) = A138840(n) if n >= 5. - Omar E. Pol, Mar 23 2018 EXAMPLE a(100) = 51 as prime(100) = 541. Concatenating the first and last digit gives 51. - David A. Corneth, Mar 23 2018 MATHEMATICA fdld[n_]:=Module[{idn=IntegerDigits[n]}, FromDigits[{First[idn], Last[ idn]}]]; Join[Prime[Range[25]], fdld/@Prime[Range[26, 100]]] (* Harvey P. Dale, Oct 10 2012 *) PROG (PARI) a(n) = my(p = prime(n), d); if(n<=4, return(p)); d = digits(p); 10*d[1] + d[#d] \\ David A. Corneth, Mar 23 2018 CROSSREFS Another version of A138840 which is older. Cf. A000040, A007652, A010879. Sequence in context: A171041 A094742 A110923 * A301378 A052424 A055398 Adjacent sequences:  A137586 A137587 A137588 * A137590 A137591 A137592 KEYWORD easy,base,nonn,look AUTHOR Ctibor O. Zizka, Apr 26 2008 EXTENSIONS New name from Omar E. Pol, Mar 24 2018 STATUS approved

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Last modified April 12 11:51 EDT 2021. Contains 342920 sequences. (Running on oeis4.)