TY  - RPRT
TI  - A First Approximation to Homotopy Theory.
AU  - E H Spanier
AU  - J H Whitehead
PY  - 1953
DO  - 10.1073/pnas.39.7.655
UR  - https://pubmed.ncbi.nlm.nih.gov/16589320/
ID  - 16589320
ER  -