A163117 The pairs of central digits of all primes with an even number of digits.
1, 1, 1, 3, 1, 7, 1, 9, 2, 3, 2, 9, 3, 1, 3, 7, 4, 1, 4, 3, 4, 7, 5, 3, 5, 9, 6, 1, 6, 7, 7, 1, 7, 3, 7, 9, 8, 3, 8, 9, 9, 7, 0, 0, 0, 1, 0, 1, 0, 2, 0, 3, 0, 3, 0, 3, 0, 4, 0, 5, 0, 6, 0, 6, 0, 6, 0, 8, 0, 9, 0, 9, 0, 9, 1, 0, 1, 0, 1, 1, 1, 2, 1, 2, 1, 5, 1, 5, 1, 6, 1, 7, 1, 8, 1, 8, 1, 9, 2, 0, 2, 1, 2, 1, 2
Offset: 1
Examples
a(1)=1 and a(2)=1 represent the two digits of 11. a(7)=1 and a(8)=9 represent the two digits of 19. a(43)=0 and a(44)=0 represent the two zeros of the prime 1009.
Links
- Harvey P. Dale, Table of n, a(n) for n = 1..2000
Programs
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Mathematica
cd[n_]:=Module[{idn=IntegerDigits[n],len},len=IntegerLength[n]/2;Take[ idn,{len,len+1}]]; cd/@Select[Prime[Range[200]],EvenQ[ IntegerLength[ #]]&]//Flatten (* Harvey P. Dale, Jul 23 2018 *)
Extensions
a(8) corrected by R. J. Mathar, Jul 27 2009
Comments