@article{JTNB_2009__21_1_109_0,
     author = {Dieulefait, Luis and Taix\'es i Ventosa, Xavier},
     title = {Congruences between modular forms and lowering the level mod $\ell ^n$},
     journal = {Journal de th\'eorie des nombres de Bordeaux},
     pages = {109--118},
     publisher = {Universit\'e Bordeaux 1},
     volume = {21},
     number = {1},
     year = {2009},
     doi = {10.5802/jtnb.660},
     mrnumber = {2537706},
     language = {en},
     url = {http://www.numdam.org/articles/10.5802/jtnb.660/}
}