1,1

This sequence has, as a subset, strobogrammatic semiprimes = A000787 INTERSECTION A001358 = {69, 111, 689, 818, 1111, 1691, 1961, 6009, 6119, 6889, 8698, 9886, 10001, 10801, 11111, ...} including the strobogrammatic square 6889 = 83^2. See also A045574 Still numbers when turned upside down (uses only digits 0, 1, 6, 8, 9 with no final 0's). A000787 Strobogrammatic numbers: the same upside down.

a(n) in A001358, a(n) =/= 10 and a(n) upside-down in A001358. UpsideDown(k) is R(k) followed by the substitutions (0,0), (1,1), (6,9), (8,8), (9,6).

19606 = 2 * 9803 upside-down is 90961 = 13 * 6997.

UpsideDown := proc(n) local dgs, a, i ; dgs := convert(n, base, 10) ; a := [] ; for i from 1 to nops(dgs) do if op(i, dgs) = 6 then a := [9, op(a)] ; elif op(i, dgs) = 9 then a := [6, op(a)] ; else a := [op(i, dgs), op(a)] ; fi; od: add(op(i, a)*10^(i-1), i=1..nops(a)) ; end: isA054047 := proc(n) local dgs, i ; dgs := convert(n, base, 10) ; for i from 1 to nops(dgs) do if not op(i, dgs) in {0, 1, 6, 8, 9} then RETURN(false) : fi; od: RETURN(true) ; end: isA001358 := proc(n) if numtheory[bigomega](n) = 2 then true; else false; fi; end: isA119738 := proc(n) if isA001358(n) and isA054047(n) then isA001358(UpsideDown(n)) ; else false ; fi; end: for n from 1 to 12000 do if isA119738(n) then printf("%a, ", n) ; fi; od: [From R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Sep 09 2008]

Cf. A000787, A001358, A045574.

Sequence in context: A056416 A061401 A046498 this_sequence A103107 A121233 A156180

Adjacent sequences: A119735 A119736 A119737 this_sequence A119739 A119740 A119741

base,easy,nonn

Jonathan Vos Post (jvospost3(AT)gmail.com), Jun 15 2006

8186 inserted by R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Sep 09 2008