polredabs not returning correct transformation map

Dear Pari developers,

Find below an example where polredabs does not return the correct transformation map. The degree 12 example is the one that fails since one should have that subst(f,x,h) = 0, however polredabs almost gives the correct transformation map since subst(f,x,h/5) = 0 so it is just of by a factor of 5. Changing the leading monomial from x^12 to x^11 gives a working example.

Thanks,

Maarten Derickx

GP/PARI CALCULATOR Version 2.9.1 (released)

i386 running darwin (x86-64/GMP-5.1.3 kernel) 64-bit version

compiled: Apr 4 2017, gcc version 5.4.0 (GCC)

threading engine: single

(readline v6.3 enabled, extended help enabled)

PARI/GP is free software, covered by the GNU General Public License, and comes WITHOUT ANY WARRANTY WHATSOEVER.

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parisize = 8000000, primelimit = 500000

? f = x^12 + x^7 - 1/5*x^6 - 3*x^5 + 13/5*x^4 + 11/5*x^3 + 2/5*x^2 + 2/5*x + 1/5;

? gh = polredabs(f,1);

? g=gh[1];

? h=gh[2];

? subst(f,x,h)

%5 = Mod(-195293748/5*x^11 + 244181236/5*x^10 - 634884076/5*x^9 + 3564400908/5*x^8 - 2539683564/5*x^7 + 166278100*x^6 - 14647209952/5*x^5 - 1269578400*x^4 + 4072182580*x^3 + 3017386300*x^2 - 1562284000*x - 7518315624/5, x^12 - 2*x^11 + 2*x^10 - 11*x^9 + 13*x^8 + 15*x^7 - x^6 - 5*x^5 + 5)

? subst(f,x,h/5)

%6 = Mod(0, x^12 - 2*x^11 + 2*x^10 - 11*x^9 + 13*x^8 + 15*x^7 - x^6 - 5*x^5 + 5)

? f = x^11 + x^7 - 1/5*x^6 - 3*x^5 + 13/5*x^4 + 11/5*x^3 + 2/5*x^2 + 2/5*x + 1/5;

? gh = polredabs(f,1);

? g=gh[1];

? h=gh[2];

? subst(f,x,h)

%11 = Mod(0, x^11 - 2*x^10 + 2*x^9 - 11*x^8 + 13*x^7 + 15*x^6 - x^5 - 5*x^4 - 5)