Geodesic
On the existence of (196,91,42) Hadamard difference sets
Kragujevac Journal of Mathematics, Tome 34 (2010) no. 1 
Adegoke S. A. Osifodunrin. On the existence of (196,91,42) Hadamard difference sets. Kragujevac Journal of Mathematics, Tome 34 (2010) no. 1. http://geodesic.mathdoc.fr/item/KJM_2010_34_1_a9/
@article{KJM_2010_34_1_a9,
     author = {Adegoke S. A. Osifodunrin},
     title = {On the existence of (196,91,42) {Hadamard} difference sets},
     journal = {Kragujevac Journal of Mathematics},
     pages = {113 - 130},
     year = {2010},
     volume = {34},
     number = {1},
     url = {http://geodesic.mathdoc.fr/item/KJM_2010_34_1_a9/}
}
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TI  - On the existence of (196,91,42) Hadamard difference sets
JO  - Kragujevac Journal of Mathematics
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EP  -  130
VL  - 34
IS  - 1
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%T On the existence of (196,91,42) Hadamard difference sets
%J Kragujevac Journal of Mathematics
%D 2010
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Voir la notice de l'article provenant de la source eLibrary of Mathematical Institute of the Serbian Academy of Sciences and Arts

We use group representations and factorization in the cyclotomic rings to show that $(196, 91, 42)$ Hadamard difference sets exist only in group $(C_7 \times C_7)\rtimes C_4$ with Gap location number $[196, 8]$. We also show that $(980,89,8)$ difference sets may only exist in four groups of order $980$.