OFFSET
1,1
COMMENTS
For an arithmetical function f whose range is a set of integers, call n a "fixated point" of f if, treated as strings, n and its reversal are both substrings of f(n). a(n) above lists the fixated points of f(n) = 2^n. In general, the faster f(n) grows with respect to n, the more digits f(n) will have as compared with n, hence the likelier it is to contain n and n' as substrings. Thus it is more interesting to consider f(n) which grows slowly with respect to n. One such function is given by f(n) = Prime(n). The only fixated points of f not exceeding 107 are 7 and 6460. Are there any more such points?
LINKS
Pe, J., Fixated Points of Arithmetical Functions [Link updated by Jason G. Wurtzel, Sep 07 2010]
EXAMPLE
2^49 = 562949953421312 in which both 49 and its reversal 94 appear as substrings, so 49 belongs to the sequence.
MATHEMATICA
Do[m = 2^n; If[StringPosition[ToString[m], ToString[n]] != {} && StringPosition[ToString[m], ToString[FromDigits[Reverse[IntegerDigits[n]]]]] != {}, Print[n]], {n, 1, 1000}]
sbsQ[n_]:=Module[{c=IntegerDigits[2^n]}, SequenceCount[c, IntegerDigits[n]]>0 && SequenceCount[c, IntegerDigits[IntegerReverse[n]]]>0]; Select[Range[1000], sbsQ] (* Harvey P. Dale, May 08 2022 *)
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Joseph L. Pe, Mar 27 2002
STATUS
approved