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In mathematics an automorphic number is a number whose square "ends" in the number itself. For example, 52 = 25, 762 = 5776, and 8906252 = 793212890625.
The automorphic numbers begin 1, 5, 6, 25, 76, 376, 625, 9376, ... (sequence A003226 in OEIS)
Given a k-digit automorphic number <math>n>1<math>, an at-most 2k-digit automorphic number <math>n'<math> can be found by the formula <math>n'=3\cdot n^2 - 2\cdot n^3\bmod{10^{2k}}<math>.
There are at most two automorphic numbers with k digits, one ending in 5 and one ending in 6 (unless <math>k=1<math>, when there are three). One of them has the form <math>n\equiv 0\pmod{2^{k}}, n\equiv 1\pmod{5^{k}}<math> and the other has the form <math>n\equiv 1\pmod{2^{k}}, n\equiv 0\pmod{5^{k}}<math>. The sum of the two is 10k + 1.
The following sequence allows one to find a k-digit automorphic number, where <math>k\leq1000<math>.
12781254001336900860348890843640238757659368219796\
26181917833520492704199324875237825867148278905344\
89744014261231703569954841949944461060814620725403\
65599982715883560350493277955407419618492809520937\
53026852390937562839148571612367351970609224242398\
77700757495578727155976741345899753769551586271888\
79415163075696688163521550488982717043785080284340\
84412644126821848514157729916034497017892335796684\
99144738956600193254582767800061832985442623282725\
75561107331606970158649842222912554857298793371478\
66323172405515756102352543994999345608083801190741\
53006005605574481870969278509977591805007541642852\
77081620113502468060581632761716767652609375280568\
44214486193960499834472806721906670417240094234466\
19781242669078753594461669850806463613716638404902\
92193418819095816595244778618461409128782984384317\
03248173428886572737663146519104988029447960814673\
76050395719689371467180137561905546299681476426390\
39530073191081698029385098900621665095808638110005\
57423423230896109004106619977392256259918212890625 (sequence A018247 in OEIS)
Just take the last k digits. Remember that the backslash means that the number continues in the next line. The other automorphic number is found by subtracting the number from <math>10^k + 1<math>.
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