login
A217613
Absolute value of difference between the n-th "emirpimes" A097393(n) and its reversal.
0
36, 36, 54, 45, 36, 27, 36, 27, 54, 45, 396, 99, 198, 792, 198, 396, 693, 792, 792, 594, 693, 198, 396, 594, 99, 297, 99, 396, 297, 693, 99, 99, 594, 198, 297, 396, 594, 198, 594, 198, 198, 198, 99, 495, 99, 99, 297, 99, 297, 297, 396, 99, 198, 297
OFFSET
1,1
COMMENTS
An emirpimes ("semiprime" spelled backwards) is a semiprime whose (base 10) reversal is a different semiprime. The first such number is 15, because 15 reversed is 51 and both 15 and 51 are semiprimes (i.e., 15 = 3*5 and 51 = 3*17). Because of the decimal base, each value must be a multiple of 9.
LINKS
Post, Jonathan Vos, Emirpimes. From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein.
FORMULA
a(n) = |A097393(n) - R(A097393(n))| = |A097393(n) - A004086(A097393(n))|.
EXAMPLE
a(1) = absolute value of first emirpimes versus its reversal = |15 - 51| = |-36| = 36.
a(2) = |26 - 62| = |-36| = 36.
a(3) = |39 - 93| = |-54| = 54.
a(4) = |49 - 94| = |-45| = 45.
MATHEMATICA
Abs[#-IntegerReverse[#]]&/@Select[Range[800], !PalindromeQ[#]&&PrimeOmega[ #] == PrimeOmega[IntegerReverse[#]]==2&] (* Harvey P. Dale, Mar 11 2023 *)
CROSSREFS
KEYWORD
nonn,base,easy
AUTHOR
Jonathan Vos Post, Oct 08 2012
STATUS
approved