

A137589


a(n) is the integer that results after deletion of all digits of nth 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
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OFFSET

1,1


COMMENTS

The plot of this sequence shows number of primes on the xaxis and the split of primes into 9 groups according to their first digit on the yaxis. The plot of a(n+1)/a(n) oscillates around 1 with decreasing amplitude. Logperiodic 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



