55XX 


Algebraic topology 

55Sxx 

Operations and obstructions 


55S05 
Primary cohomology operations 


55S10 
Steenrod algebra 


55S12 
DyerLashof operations 


55S15 
Symmetric products, cyclic products 


55S20 
Secondary and higher cohomology operations 


55S25 
$K$theory operations and generalized cohomology operations [See also 19D55, 19Lxx] 


55S30 
Massey products 


55S35 
Obstruction theory 


55S36 
Extension and compression of mappings 


55S37 
Classification of mappings 


55S40 
Sectioning fiber spaces and bundles 


55S45 
Postnikov systems, $k$invariants 


55S91 
Equivariant operations and obstructions [See also 19L47] 


55S99 
None of the above, but in this section 